I have a sister who is 7 years older than me. When I was young, she was generous enough to allow me to hang out with her frequently. We'd go to the mall and she was sweet to buy me a pair of jeans or a t-shirt. We'd hang out and watch a movie. I was never a terribly popular person, but hanging out with my sister always made me feel more accepted & cooler. (Heck it still does
) She'd been there before me, and had great advice or at the very least could listen well. Although you aren't going through exactly the same challenges, your sister is facing some of the same things that you've already dealt with.
I think that one small thing you can do is to try and make some time for her. You say you don't get along well, but even my sister & I fought and annoyed the crap out of eachother. And besides, who better to complain about your parents to than a sibling? They've been there!!! Maybe you could offer to take her to a movie she wants to see. Not out of pity, but because she's your younger sister and you'd like to get to know her for who she is. I know sappy... but it's the truth. And the fact that you care enough for her to write this thread shows me that.
Along those lines I still encourage you to talk to your parents about taking her to a therapist, or at least have her keep seeing the counselor. A professional could not hurt in a situation like this.