Firstly, you definitely aren't alone. Even just looking through the archives here there are a lot of girls and young women who feel stricken with guilt over doing what you also did as young children. You aren't a freak or a monster by any stretch of the imagination.
Also, it is important that you were a young child. That doesn't mean that was you did was totally okay or not worth addressing, but it definitely means you should be far, far kinder to yourself now. The fact is, on an objective, scientific level you are *not the same person* at 19 as you were as a 12-year-old. In those seven years your brain has changed and developed in countless ways and will continue to do so for a few more years. You should judge yourself as you would another young child of a similar age. What you did was regrettable, but it was not done with an adult's understanding or morality, nor an adult's self-reflection, self-control or maturity. You should not have to be forever chained to the actions of a 12-year-old girl. You were a kid.
Now you are a young adult I hope that - to some extent at least - you will be able to put a line under the sand at your 18th birthday. International law and increasingly the cultures of the world recognise 18 as the point at which a child becomes an adult. These actions belonged to your childhood. They do not define you as an adult.
As for bringing this up with your family, I think all you can do is to be open to the possibility that one day your brother might bring it up with you (though I think it is unlikely that he will ever remember TBH). I think telling him outright would actually do more to confuse, upset and hurt him than anything else, especially since he is still very young. I also feel that you doing so would be more to assuage your own guilt and anxiety than to help him.I think the important thing for you to do right now is to be more compassionate with yourself and find a way to live with your guilt and anxiety.
Personally, I feel therapy could prove really helpful for this. I'm not convinced you would necessarily be reported, but I live in Britain not America and we may well have different rules regarding this. At the very least, you could seek therapy and just tell your therapist that you were harmful or inappropriate with your brother when you were little, without going into details. I definitely think this can be addressed in therapy without you ended up in prison, which I think is incredibly unlikely and also would be utterly inappropriate and not whatsoever helpful in your case.
You're not a monster. You're a decent, self-reflective human being who did something stupid and regrettable as a young kid. You absolutely have a right to life and you can find a way to live with this. The best thing you can do is to learn to live with yourself and be a positive force for good in the world and the lives of your family. I strongly believe you can do this!