I think therapy is a good idea TBH even with the risk of the authorities being contacted. This won't necessarily happen and I feel - even considering this risk - you need to have a space to work this through and to ensure that you won't ever act like this again (I believe you are in good faith when you say that you will not, but it's also about taking the necessary step of therapy to absolutely ensure this is the case, it demonstrates your commitment to both yourself and society).
I must admit to finding your post a little confusing at first (which is why it's taken me a couple of days to respond - so, my apologies for that). Were these actions of yours definitely performed with a sexual intent - or, indeed, with the intent that there would be genital contact? I ask because when younger I played the airplane game of holding a younger sibling up with my feet and while I was not whatsoever aroused doing so, I suppose if I had have been, my sibling falling off my feet (as invariably happens after you lose the balance) and onto me could have involved contact through clothes like you are describing i.e.
was it that you deliberately made her fall onto your groin or that when she fell she happened to do so and you are worried that you found the experience arousing?
The most concerning things you said are IMHO you masturbating behind your sister's back and - //if it did actually happen// - you grinding your head and feet against her groinal area, as I would agree that these things constitute sexual abuse.
I hope you can take some comfort from the fact that you were still a child yourself, albeit an older one who should have known better. To some extent I think you should be able to draw a line between yourself as a child and yourself now as an adult and not consider yourselves the "same" person. Also, the fact that there was no direct contact involved (or, rather, it was through clothing) makes it more likely that your actions won't have a long-term traumatic impact upon your sister, which is the most important issue here.
I don't think you're a monster or evil or even a paedophile - just a thoughtless teenager who did something pretty messed up. I think therapy would help you work out why you did what you did and would allow you, in the long-term, to get some closure and move on with your life.
I am sorry if this post is a small comfort and not as reassuring as you might have hoped for... I want to be realistic and fair, so I hope you can take what I've said at face value because I'm not writing this feeling anger or disgust, but what you did makes me sad and I think it was wrong, though not such a wrong that all is lost!! It isn't. I think, in the long-run, you'll be okay. Have hope and take heart. You are, at the very least, no longer the child that you were. And I believe you when you say that you'll never do something like this ever again.