I am so sorry your mom is so cruel. My best friend's mother is very similar. Ignore her and if your siblings don't want to discuss it &^%$ them too. Make plans with your kids and enjoy the holidays. You don't have to put up with being treated this way, and you sure as hell are not obligated to expose your children to this behavior.
HUGS to you. I know it must hurt. I would be incredibly angry too.