My brother moved in with my 86-year-old father a few years ago along with his son and boyfriend. My disabled nephew also lives with them. My brother and boyfriend fight all the time and his boyfriend is verbally abusive to everyone. Very mean and nasty. My brother is away and he is there making everyone s life miserable. My husband and I can't go over and visit because of him and a dog that likes to bite. My father doesn't want to do anything for fear of upsetting his son, but something's got to be done for everyone's well being.
If all else fails, call the police if it gets out of hand with the verbal abuse. Verbal abuse always precedes physical abuse.
Like Scampie suggested, you can report him to APS but it's possible he will just shoo them away in order to not upset his son, so don't have any expectations that this will solve the problem.
You *can* go over there and visit, you just don't like doing it because of the bully. The best way to deal with a bully is to not allow them to bully you. This takes away their power. Go over there and if the bully starts in, then start videoing him and the whole situation. If the house is a wreck, or your Father looks unkept, video all of it. If the bully threatens you or anyone else, call the police immediately.
It's your Father's house and he gets to decide if you visit there or not, not the bully. If the dog is the issue, go prepared with dog repellant spray (chewy.com or get the police/military strength spray). Tell the bully that if he doesn't put the dog away in another room you'll spray the dog if comes nears you in any threatening way.
At this point if the other "tenants" are having their mail delivered there, then it is their legal residence and they'd need to be legally evicted (each one separately).
Step away. Unless your father is incompetent in his own choices this is his decision. It sounds as though brother, who is "away" is working; boyfriend is caring for Dad and maintaining the household, and nephew, disabled, is receiving care as well.
Instead of adding complications, offer help.
ie: "I am going to the grocery; can I bring you anything" . Taking over a bean casserole. Offering to mow the lawn, plant flowers. Take Dad out to lunch. Whatever you can contribute.
People have differing ways of acting and interacting together, and what sounds to YOU like abusive language may just be "their way" of speaking. Don't stay around to listen to a lot of it.
Keep a good solid relationship with your Dad and when he is unhappy is the time to worry about all of this.