PP's have given excellent suggestions for how to deal with BIL in the moment
while he misbehaves.
...this is a level of dysfunction I could not handle. It would be too mentally exhausting for me. OP, you have a DH problem, not a BIL problem. I would suggest that you set the pattern for how to command respect by refusing to your DH's dictates that you must be around this unpleasant person.
Sweetly decline while letting him know you understand that he wants to keep a relationship
with his brother. At some point, your DH may have an epiphany that in keeping a close relationship
with his brother, he's giving up important milestones to be spent with his wife (which, when you marry, your spouse becomes your primary family - not your FOO). Also he may note that you are mentally healthy enough not to put up with mistreatment, so why isn't he?
Best wishes to you OP!
ETA: Oops, just read your update! Good for you. Agreed with SoCalVal, though:
Quote from: Susan45 on Yesterday at 02:59:03 PM
My husband will try to talk me into still doing his birthday dinner, I guess, but I'm not going to. Something about a few comments I read that said I shouldn't have to cook for someone who has been mean to me and abused me. He was so fake around my husband. I am not putting up with that anymore. I can't believe what I took off that person. All because I love my DH and didn't want to make waves. Come what may, I feel so good about having confronted him just now. I don't care what he thinks of me. I couldn't care less.
Again, good for you.
What is up with your husband? We read that you love your husband, but how much does he love YOU that he would try to make you do this still??? Is your husband so afraid of your BIL that he won't do anything that will displease him and, really, risk his marriage?