I really don't know if I have any brilliant solutions to your current problem. I'm glad that your bf did not drink the other night when his ranting father showed up on your door. Personally, I would have called the police once the man started.
Is there any way you can limit your time with your boyfriend's family? Again, this is just me, I don't think I could purposefully place myself into a situation where I knew that people did not like me and would treat me with contempt--just to make someone else happy. You deserve to have some peace and joy this holiday, too!
In the past, when I have been confronted with angry and hateful people, remaining calm and detached has worked quite well to diffuse the situation. Humor, not reacting, not raising my own voice to match their's, all go a long way to calm things down--plus then the abuse they are dishing out is apparent to others as well. That and perhaps stick as close as possible to the members of your boyfriend's family with whom you get along really well!
Good luck with whatever you decide!