I don't think you're mentally ill or a bad person. Good, healthy people, can make bad choices. However, isolated (even some ongoing) issues don't define what we are by a long-shot.
That's what you did...you made a bad choice out of confusion and desperation. It felt good and you indulged...like with any coping mechanism or "escape". Once it went so far, it was very hard to give it up for many other reasons as well.
I'm not saying I think what you did was "OK". I don't. I do think you've recognized it as not good and changed it...which is great!
You've ended this, and now it's time to forgive yourself with the understanding, maturity, etc. that you've gained, and move forward.
We all make mistakes, sometimes doozies, but if we're lucky we learn from them and apply them to our life experiences. It sounds like you're doing just that. Way to go!
I'm glad you've sought help for your depression. Are you in talk therapy as well? I think it would be great for a little while...someone all your own to understand...
I'm sorry life is rough at your house. I'm sorry you feel the "differences" made between yourself and your sister. Soon you'll be off to college and answering to yourself, in control of your own experiences.

From your post, I've no doubt you can be anything you want to be.
KD