Because you are troubled doesn’t mean you are insane. Questions like yours are complicated. Generally, when someone tells me that a kid under 12 was molesting a younger kid, I’m concerned about both. I’m concerned about the younger child because she was touched inappropriately by someone she trusted and loved. I’m concerned about the older child because kids generally don’t know about sexual behavior at that age unless someone older was molesting them. If that’s the case with you and your sister, you were both victims of someone who hurt your sister, who then acted out her confusion by hurting you. Looking back, it’s fair to guess that she is as troubled as you are.
For that reason, I hope you will scrape up some courage and have an honest talk with your sister. Let her know that you know that what was going on was messed up but that you also understand that she was a kid too. See what she has to say about it. She has been carrying this secret too. It may be a relief to her to have the chance to talk about it. Or not. But you won’t know unless you try.
An honest conversation is a much healthier way to deal with your pain than cutting or hurting yourself. Hopefully, you’ll be able to get some closure on your past so you can turn your attention to your present and future. If you and your sister have difficulty talking things through, please consider seeing a therapist to help the two of you. It’s possible that the two of you can be an enormous support for each other but you don’t know how. A therapist can help you get to a new place with each other.
I wish you well.