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.
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.
If all else fails, call the police if it gets out of hand with the verbal abuse. Verbal abuse always precedes physical abuse.