Honestly, the brother and mother make me angry. They watch her beat herself up, grind herself down, and just don't suffer any consequences for it. Especially the mother. Her insecurities are basically all due to her mother's blatant favouritism, leaving her daughter hating herself, and somehow that's all just fine and shit.
And the brother too, if he's worried about his sister, don't just take her on some trip, TALK to her!