Unfortunately they come as a family, and you can't exclude him without causing big headaches, its just one of those things.
We are having the same sort of thing with step sons current girl friend,
we are a large family, and all get together at least once a month,
this girl completely spoilt our last night out, she is the most objectionable, obnoxious female I have had the displeasure to meet.
Everybody tried so hard to include her, make her welcome, etc, but I don't know why we bothered.
Am I looking forward to the next family get together....NO
Am I going to try to arrange it last minute, for an evening when she can't come....YES
Am I a horrible person.....I think I must be. X