I wish I could give you a definite answer to your question. I can’t given the limited information in a letter. What I can do is validate that you are troubled and that you need and deserve some help.
Your feelings probably have something to do with your relationship to your sister and whatever caused your parents to seem to favor her over you. The kind of situation you describe sometimes makes a kid do lots of things to make it clear that, yes, she exists too. Feeling so invisible and powerless, you manipulate people and hurt yourself to tell yourself that you do have some power. Although it does help you prove to yourself that you have some control, it isn’t healthy and won’t lead to a positive self-image or a happy life.
If you could have found other ways to feel like you have a place in the world, you would have done it long ago. For that reason, I strongly urge you to get into therapy. Ask your school nurse or guidance counselor how to find a therapist who works with teens and their families. Your whole family needs some help — not just you.
In the meantime, if you need someone to talk to, please call the Boys Town hotline. Don’t be put off by the name. Girls can call too. Counselors are available 24/7 all 365 days a year to talk to teens who are distressed or scared or confused. It’s absolutely free and confidential. The phone number is 800 448 3000.
I wish you well.