Your dh does NOT need to tell mil anything in advance in my opinion. Talking had not done any good so far. And from my limited knowledge of catholic mass via my friend who volunteers for lots of organizational things at the cathedral she attends.. Christmas eve masses are for Christmas eve, the special masses for deceased will be held at other times. And your dh called and spoke to the church and mil is still trying to convince him that something will be happening that just is NOT happening.
Spend time enjoying yourself.
My mother shared with me, many years ago, that before I was born she and my dad went about three years not speaking to dads sister and her husband, because her husband was just so obnoxious. Eventually he got a bit less obnoxious and they started speaking again.
I agree with others. As long as your dh chases after his brother, nothing will change. Your bil needs to just be ignored, and that includes no acknowledge ment from y'all and as little contact as possible. If he brings a gift to mil house, leave it there or throw it our without opening out, and make sure mil sees that. If you want to be nice let your kids give their cousin a gift.
Help your dh practice broken record technique. ” mom, we are not coming over Christmas eve. ” only say that, no discussion, no justification, no apologies, no excuses, don't get defensive. ” mom,.we are not coming over and you know why. I am not discussing anything with you, it won't change anything. We will see you another day.”
Edited to add: Cicero has very good advice. Practice that conversation with your dh.