Honestly, how does she think that this is ok behavior? By the way she has written her posts it is very clear that she sees nothing wrong with her actions and for some reason just can't possibly fathom why her family has excluded her from Thanksgiving and is slowly cutting her out of their lives.
The OP, u/Psychological Bid58, has made multiple posts regarding her relationship with her sister's ex-husband (and the father of her children). This guy cheated on her sister when she was at home with their new baby and for some reason, he seems to be prime dating material. The first post is the most recent AITA post about their family vacation. The second is a recovered deleted AITA post. The third a r/relationship advice post she posted about her relationship with this guy.
If you want to check out another complete piece-of-work check out this woman: Woman Demands That Her Fiancé Publicly Support Her Psychotic Lie About How They Met