I honestly think there were a lot of reasons
a combo of all of them
the history with her dropping out from her pregnancy , her husband , her being the only chunkier one in adulthood (til Trina discovered pizza and wings) .... There was a lot of things which separated her from her sisters in one way or another .
It's almost inevitable there was gonna be a certain tea in the air.
So I can't even say they were PURPOSELY being mean to her or anything or that she was "singled out" by her sisters . The universe kinda singled her out