Just went back to chapter 1 to see how her sis treated her.
Her sister was a really young girl who didn't seem to understand what was happening to her older sister. When Paulina was dragged away, her mother seemed to tell her to ignore it, and the little sister seemed very confused.
I think she would've been too young to mistreat Paulina and was probably raised to ignore her. After Paulina left, the parents also probably groomed her into thinking of Paulina as a evil bitch.
I really look forward to how this will play out! I really hope that the sister somehow miraculously becomes friends with Paulina, that'd be a refreshing/new concept!