I'm with Katchie on this. Try booking a time to sit down with your husband and without getting defensive, listening more than you talk, see if you can come to a compromise.
Don't forget your H has known this guy his whole life, remembers him when he was young, and has no doubt seen their late mother enable him. He possibly thinks that's how it works.
You two might be able to come up with a compromise which neither of you is thrilled about, but will save your marriage. Even talk to a counsellor together. Just don't let your BIL threaten your marriage.