You and your son need to spend Christmas WITH HIS DAD. Your fiance' and you are your son's family, the rest, is extended family. If your mom is ok with being bullied into seeing her grandaughter, that is her choice, but it comes at a price of missing out on her grandson... If this were the case, and you were to follow thru, I bet your mom would sing a whole other tune. She is all about keeping the family together, I doubt she would dance to your sister's beat once you start keeping YOUR family together...