How does someone explain diabetes to another person who doesn't have it?

Who or what is the "relation" that you are talking about?
A spouse? a Significant Other? a sister, cousin, child?
You really can not explain to someone that has no interest in doing what needs to be done. Even if they went to a class where this was all explained if they do not care to follow through there is not much you can do.
UNLESS..
If you think this person is purposely giving BIL an unhealthy diet.
If you think BIL is in danger of losing his life (a real possibility)
You could report the "abuse" to APS. This is a long stretch.
You could talk to a lawyer about trying to obtain Guardianship. (Do know this will cause a BIG rift in the family)
OR
You wait until there is a crisis and then try to step in. (if he recovers from said crisis and is not one that goes into a Diabetic Coma and dies in his sleep.

You might also discuss with the "relation" how much more difficult it will be to care for BIL when he is a double amputee and bedridden.

How old is BIL? If at an age where nearing end of life, then fighting this battle may not be worth the effort. If BIL is younger, then find a simple explanation of diabetes and how it impacts the body and give this to other relations.

Ranting is a relief, this is a good place for it. Is your spouse involved in this situation, too? Can he be an ally? Maybe trying to set a limit - in our house, we serve the food, please do NOT bring any food to our house. They will push back at any kind of limit you set, but that might be one that could be enforceable. If they bring food, take it away and return it to them when the visit is over.
Trying to control blood sugar with diet alone is challenging even without this issue. Ask the doctor if there are medications he could take that will let you ease up a bit on this worry. Often diabetics who start with diet control progress to meds as they age, anyway.
Take care of yourself. You are a good person, to take on his care.

Talk to his doctor or look up a diabetic association online They will have information in easier to understand formats. Ask all of the family to review the information with you. Then, discuss together how everybody can help BIL stay on his diet. It may take a bit of conversation and brainstorming to come up with solutions to the "let him eat what he wants" to get same understanding of "what can we substitute for a high carb food" and "what can we do other than eat to show love".

Maybe you could try explaining to them that his diet will kill him if this keeps up. Maybe they do not understand that.

Give them the Dr's Orders and a Booklet in it.

Who does BIL live with? Having had a diabetic husband and mother, I know learning the diet is a process. Until my husband was diagnosed with kidney damage, I did not treat the disease as a family one—which it is. And everyone benefits from following a diabetic diet—whole —not processed (like pizza rolls) —foods, whole grains, portion control with many small meals rather than a couple big ones, and yes, avoiding (or carefully limiting) the high carb items—fruits like citrus, grapes and raisins, watermelon, pineapple and “the whites”: white bread, pasta, rice, potatoes. Also high is corn (starches are sugars). The more fiber in something, the better.
It can be overwhelming at first. I recommend appointments—more than one—with a nutritionist. The doctor can write a referral. The nutritionist is the most important person on the diabetic’s care team in the beginning. It’s too much to try to learn on one’s own, even for the capable and motivated.
The family should attend the sessions. And it’s not just that eating the wrong foods will kill him. It’s the quality of life before he dies—with more energy and all his tors, both feet and legs, his eyesight. You are right to be concerned. Getting through to those responsible is his best hope. And making sure the house is filled with the right choices will help everyone not feel deprived.
Best of luck on this journey.

It's just like your teeth, except too much sugar weakens your whole body. The effect builds up slowly. Once your body loses the ability to turn sugar into energy, you can't get it back. (Just like teeth.)
*Every day*, you have to take medication (fillings). It can compromise his walking so you need a wheelchair. Or puts his eyesight at risk. If it gets really bad, you need a needle everyday (root canal).
They're all true.
Even if you have to repeat them, substituting sugar makes it easy to remember. Parents probably did it with their kids.
One meal at a time.
Good luck.

High blood sugar causes nerve damage and circulatory damage (damage to capillaries).

Nerve damage won't kill you but it can cause unremitting and often untreatable pain and, due to the risk of inadvertent injury because sensation has been lost, disability.

Circulatory damage leads to edema and contributes toward diabetic sores (often on the feet or legs) which may lead to amputation.

They may be thinking "Well, it's better he live now as he wishes" even if he drop dead a little sooner than he might have otherwise. But if they realize uncontrolled blood sugar causes irreparable damage that is far more likely to lead to serious quality-of-life issues rather than sudden death (years of increasing pain that often progresses to disability many years prior to eventual death) they might have a different attitude.

Babs, you might try searching for a diabetic forum, find some to the point posts, especially about noncompliance with diabetic requirements, and share them with him and the noncompliant relatives.   I think sometimes it's hard for diabetics to believe that such drastic complications could affect them, but it happens.

You might also find a diabetic support group, especially a hospital sponsored one, and either take him (if he's mobile) or share some of the discussion issues if you have time to attend.   

Or if you really want to be forthright, print out information from either a diabetic association or some of the major large hospitals on the challenges and hazards of noncompliance, gently reinforcing to him and the relatives that diabetes is nothing to treat casually.

I hope you have luck in this; I know it can be hard.   I have relatives who've already been diagnosed, gone through extended recovery periods, but also still ignore the fact that they're playing with fire.

If he has dementia and on top of that is stubborn, you can try to fix this until the cows come home - but it won't be fixed. So, you either REMOVE OUT OF SIGHT AND NO ACCESS for all food not to be eaten by him - Others have to help too or get out of the kitchen. If all fails, it will catch up with him and he will live with the bed he has made. End of story.

I had two friends with diabetes, one a juvenile diabetic, type 1 and one who had it later in life, type 2. Both had a leg amputated and both passed because of kidney failure, 63 and 69 years old. Diabetes effects every organ of the body. Because bad circulation in the legs happens, then they get sores that if they won't heal turn gangrene and thats where the amputations come in.

Its just not the carbs in those pizza rolls but the Salt totals 1850 mg. Average person should not use more than 2000 mg a day. Salt and insulin retain water. So the more sugar and carbs you eat, the more insulin you need the more water u retain. This relative is killing your BIL slowly. Your BIL should be eating good foods and keeping his weight down. Not that he can't have a treat every so often but better to control this with diet than to need insulin shots every day.

There is a difference between type 1 and type 2. I suggest you call your local Hospital and see if they have diabetic training available. Then u, BIL and relative attend it. You really need to understand diabetes to be able to control it. There are some fruits that should not be eaten because of the natural sugars. Carrots are a vegetable with natural sugar you should not eat. You need to know the signs of not enough insulin, too much insulin and low sugar levels. My girlfriend carried a small can of fruit juice with her all the time. Some diabetics carry jelly beans. Some diabetics get lethargic and need hospitalization.

This is a serious desease.

Babs2013: Imho, perhaps you can tell "his relation" just how serious a disease that diabetes is and these individuals will be better equipped to handle it.

If his short-term memory is gone you can't expect him to follow a diabetic diet or even understand one. "Counting" carbs will be an exercise in frustration for you and pretty much a wasted effort for all involved.

Are there low-carb foods he enjoys? If he likes "salty" things try giving him some dill pickles to snack on. There are some good sugar-free desserts (puddings, frozen yogurt) available. And low- sugar bottled nutritional drinks can be satisfying. Maybe some beef "jerky" could take the place of pizza rolls. Celery stuffed with soft cheese might be good for munching and help keep him feeling "full".

Ask the physician for a dietitian to spend some time with family to suggest appropriate foods he might enjoy. Then stock up on low- carb substitutes that he might be willing to eat. If he is elderly and forgetful, you may have to settle for keeping him full with low carb nutritious foods rather than continually trying to follow a traditional diabetic diet. If he has a visiting nurse or hospice they can check his blood sugar from time to time (or, if it worries you, you can, with a simple home glucometer--get a health professional or diabetic friend to show you how. Have a nurse or doctor explain to you how to interpret)

When the diabetic coma hits...reality will sink in. Sorry to say.... but I have seen it happen.

My partner with dementia refused for years to take his diabetic meds and he ended up with nerve pain in his feet. It felt like his feet were on fire. Along with the dementia he had delusions, one of which was that someone was sneaking into the house to set a fire under the bed. There was no proving to him that it was not true. He would yell, scream, threaten this invisible assailant, acuse me of letting him in. This was every, single, night. For three years, I did not get a full night's sleep. Diabetes is serious and with dementia a double whammy, not just for him being in chronic pain but for those around him. Do whatever you must to get his sugar under control. I would not wish the last few years on anyone.

My Dh has Type 2 diabetes and has been on a roller coaster since the dx (5 years?) I read about it, got all the info from a niece who works with diabetics specifically--and really revamped what foods I kept in the house.

For a little bit, he was compliant, because he was scared. Then he got lazy and just did what he wanted.

Then 2 massive heart attacks scared him back into compliance. For a while. Then he 'got better' and started just eating whatever, again.

Last winter he finally tanked, and it was due to the uncontrolled diabetes. He has done some permanent damage to his body and we are working to repair what can be repaired and trying to be better. (I can say that I was NEVER the problem. He is a grown man with all his faculties and simply thinks he's invincible).

NO food is totally off limits---as long as you eat in moderation. Once you tell someone they can never have something ever again, the normal response is 'oh yeah, watch THIS' and they'll indulge.

Dh spent a week in the hospital as they ran umpteen tests on him and the final dx was the diabetes. He always maintained that since it wasn't type ONE, he was fine. Oh, no, not true.

I told him if he would not watch his diabetes and get it under control and KEEP it there I would no longer live with him. I cooked healthily, keep only 'good foods' in the house and some diabetic friendly snacks and he'd routinely sneak out at night and get burgers and shakes at McD's.

He's been better. He wears a monitor on his arm and checks his blood sugar 6+ times a day. Has his A1C down to where the doc is finally happy.

As a family---we still eat the way we always used to, but he avoids the breads, chips and 'hi carb, low quality' foods.

Will he stay this way? I have no belief that he will. But I always hope. Just the neuropathy in his feet should be bad enough to remind him to 'behave'.

As far as your family bypassing the 'rules'---people equate food with love and it seems against our culture to not bring a treat of some kind when coming to visit. My kids know they can bring treats to the house. BUT, they have to take the leftovers back home.

I wrote and sent an email to my kids and explaines exactly what dad should and shouldn't be eating. This was not news to them, 2 are doctors! But they knew that I would appreciate them being thoughtful and supportive.

Honestly, with an adult, it's hard. DH has always done exactly what he wants and threw caution to the wind, always. Now, at age 70 he is reaping the life of excess and incautious living. Sad that his LO's also have to suffer.

--I think your situation is so much worse--since you can't really control BIL and you obviously care for him! Try getting his doc to approve the Freestyle glucose monitor--you attach it to your inner upper arm. No needles, readings are picked up on your phone. This, for Dh has been a game changer b/c he's a really techie guy and loves to show me his 'readings'--and he can see in real time how he's doing. Maybe your BIL would take pride in keeping his BS in line when he can see on a constant basis what's going on.

I know Dh looks at me, with 40 lbs of 'cancer fat' that I can't shake and feels very angry b/c he thinks 'I' should have diabetes. Life's not like that, is it?

(Your story of BIL eating all the pizza puffs remind me of DH eating an entire 'family size' pkg of Oreos one night. 30 servings. I think some people have no 'shut off' valve.)

Regrettably, because he has dementia, I matter what you tell him, his memory and understanding is compromised.
Think back when you were a kid, mom or dad would tell us we can't have a cookie before dinner. Many of us would try our best to sneak that cookie, just because we were told no. People who have dementia, or memory issues seem to present with the childlike responses as they don't have the ability to make a good decision.
Many don't even remember they don't like certain foods.
Whike the family and friends should be helping, from the sounds of your comments, they are not helping, but compounding your concerns by only doing what they hear him saying he wants. "The perverbial cookie".
Whike my comment may sound a bit brash, I would probably say something to the effect, "Do you hate your _______ so much you want to kill him"? And explain that these foods they want to keep feeding him will lead to either his untimely death or suffering from amputations, strokes, neuropathy, blood clots, hearing loss and a slew of other things and they would be the cause of his health decline. Then say, do you want to be the one that makes him suffer or worse die?
One thing I have learned over the years is there are some instances where no matter what you do or say, it won't change anything 😞
I wish you luck, but as far as the brother in law understanding, that boat has sailed.