Unwelcome visitors

Alas! If a witty riposte or soulful rebuke could shut down a drunken bully, history would have played out very differently, my dear. Since there are no magic words, you must speak to your friend. She (you didn’t mention gender, but I’m going to say “she” to distinguish host and guest) ought to know that her relative is acting up. It’s the responsibility of a host to ensure that everyone at a party is behaving appropriately, and her brother-in-law is certainly not. Don’t be accusing or angry, but describe his behavior as matter-of-factly as you can. If your friend responds with some vague hand waving, let her know that as far as you can see, your options are to continue being bullied, stop attending her parties, or assert yourself in a manner that you would judge rude and she might find upsetting. If she continues to waffle, simply stop going. But if it does get to that point, try to be forgiving. She isn’t necessarily choosing the bully-in-law over you; families are complicated, and she might be more constrained by family politics than you realize.