((HUGS)) I know how you feel. I walked into the court house and seen a close family member (of mine) standing next to my ex ready to testify against me...broke my heart. I used to watch his little girl all the time (were talking 16 hours while he worked), when I had told him about my ex and the gun issue he was the one who told me "Call any time I'll be there. Family sticks together"
It is up to you how you deal with it, me...I no longer think of this person as family, but it doesn't mean it hurts any less.

I for one would not be so excepting of my ex being at family dinners and such, but then it would make for an interesting time with my Daddy and my brother there too

OMG its bad enough for my ex's family whom i catered to and put up with their cruelty to turn against me when my husband cheated and left us, but to have someone in my family do that would be just horrible! I dont get it. my heart goes out to you, hang in there!