She knows she can always come home, but I think she sees her sister's place as a sanctuary where she won't get judged. Of course all of this is incredibly selfish, but it's what she chooses.
Do you see why she would choose to live there?
Make your home safe for her, make it desirable for her.
M22,H45,W45 S21/18D12 Retain faith that you will prevail in the end, regardless of the difficulties and at the same time confront the most brutal facts of your current reality, whatever they might be.