Lizzy: I think that I know what you are talking about, but here is my take on a similar issue that I have had. My son in law feels very free to intercede between me and anyone else, a waiter or a family member when I confront the waiter or family member. My son in law evidently confuses me confronting someone's bad behavior towards me as being a jerk to them, which is not the case. I have a right and obligation to stand up for myself and call a waiter on bad service or others on mistreatment of me--albeit in a calm and respectful manner. Regardless, I don't think that it is any of his business how I interact with other adults. If THEY have a problem with it, that is for them to tell me, not him. If I were being nasty to him, then he has a right to confront me. Or, if it concerns underaged children or others who need protection, then he would be right to step in if I were being abusive. But that is not the case. He just feels free to censure me because he thinks confrontation is abuse, which it is not and that it is his job to protect others, which, again, is not his responsibility.
But, I assume that your father's mistreatment of others is, first of all, actual mistreatment and second of all involves adults other than you. I am going to ask some pointed questions next, please forgive me, but here goes: If that is so, why is that your concern? Why don't the others say something in their own behalf? Does it just make you uncomfortable? Or judgmental? If it were me and I wanted to address the issue, I would do it at the time it occurred, which is difficult, but better. For example, if your dad said, "Gwendolyn! You're an idiot!" to my aunt at Christmas I would say, "Dad, I really don't like it when you call people names like that because it upsets me." (When you do such and such, then I feel like so and so, not, "Dad, you have no right to say tha to Aunt Gwen!") He might come back with some excuse or defense, but just say it again and again until he shuts up. He really cannot argue with how his behavior makes you FEEL. It might not change his behavior, but it would call him on his crap and you would say how you feel. It might encourage Gwendolyn to call him on it also, and not put up with it. And the whole rest of the group! Lead the way! They seem to be in agreement with you. I think this would allow you to stay in the relationship on your terms. If you are relieved to let this relationship go, then maybe you should. But as for me, I always see, first, if I can have the relationship on my terms, both respecting "Dad" and myself, saying how the other person makes me feel (which I have a right to feel and to say), and reserve a complete cutoff from the relationship because I decided to do it, not because I left up to them.
Good luck with this! I know it is a knotty problem. I hope you can have peace about whatever you do, and I wish you the best! Honor yourself however you can and however you have to do it!