Tough family dynamic
Every family has its issues.
Holidays often magnify the underlying problems. There is no right answer for this one. Smiling and being nice often diffuses the other person who may be trying to pick a fight. It sounds like you and your sister need to "talk it out" but christmas may not be the day to do so. Rem, if you get mad then she wins. Go to dinner, smile and try to not let her get to you.