Firstly I'd speak to your parents or a teacher if possible and explain what you've explained to us. It's nice to be nice but some people are toxic and attention seeking and as you say you are not a therapist, and what she is doing is manipulative and wrong. It's not possible to genuinely support someone who is controlling you, so you have to either break free completely or set firm boundaries and stick to them, but certainly her parents are behaving appallingly putting this onto you, they need to be helping her find a way out of this toxic behaviour and finding out why she is so unhappy as to engage in it in the first place. x