I am not catholic, but if you confess this to a priest you will feel a hell of a lot better. What you need to do is to understand that you are not a freak. You are not, you were simply horny. You didn't DO anything wrong. You COULD have done a lot wrong and scar her for life, but that didn't happen. You need to have compassion for yourself and forgive yourself for what happened. You can also be super extra nice to her as ways to repay her for whatever small damage may have happened. Finally you can also write this and more more more more in a journal and you will feel a lot better.
Don't be so hard on yourself! It was a childish act born from sexual awakening that we all go thru.
I am sure if you discussed it with your sister she would be amused at the level of your concern.
Forget it! Buy her dinner as our pentence and don't mention it or worry about it ever again.
I bet you any amount of money that she doesn't even remember the act. And if she does, I bet she's either not concerned or has see/done worse.
I personally forgive you. Let it go and live your life.
You shouldn't be so hard on yourself, you did nothing wrong, you were a child with a natural curiosity about sexual sensations and feelings which is nothing more then normal and natural human sexuality and should not be considered perverted, sick or Stigmatized morally or mentally as some kind of defect. If anything you did right by leaving her alone when she pulled her hand away(indicating that she didn't feel comfortable with were you placed her hand)instead of forcing her to touch you which wouldn't be right at all and in any case, children as well as adults are more likely to suffer negative effects from a forced molestation or rape then if they had agreed and wanted to participate in the acts themselves, I'm wondering if you're harboring feelings of guilt and shame solely because of what you were taught by family or society. I hope you can come to the realization that you're probably the only one who needs to learn to forgive your own conscience for something you've done that doesn't even constitute forgiveness.
When I was 12 I raped my 5 year old sister. I repeatedly did this until I was 15. That was over 40 years ago and she has not spoken to me in over 40 years. In fact not one of my very large family has spoken to me in over 40 years. At 15 I was caught, went to prison and got out a few years later. A decade ago I found out my mother died by reading her obituary in the newspaper. What I did was monstrous. What you did is not even worth feeling guilty over. You did absolutely nothing of consequence. Stop beating yourself up over this. Just be happy for what you have. If you still feel guilty just do nice things for your sister. Always be there for her and never tell her that anything you do is out of guilt and never discus this with your family. It just isn't nearly as bad as you think it is and most people won't understand your confession if you choose to confess to anyone.