Your sister is the only person you might need to talk to about this. It happened between you and her. I don't think it's as much of an issue with her as it is with you, tho. I think you should only apologize if she starts to behave as if she's afraid of you. Otherwise, forget about it and let her get on with her life. Talk about it when you're both adults and neither of you are in any danger of going to lockup for your tiny indiscretion.
Xena, I disagree entirely. My family is suffering currently because of the same issue. My husband abused his sister 13 years ago, admittedly it went on for about three years until he had just turned 16, she was 8 when it ended. He didn't tell anyone, until me last year. He hadn't spoken to his sister and had assumed that she was quite comfortable with him being around. A little over a month ago she let us all know very clearly that what he had done to her had affected her, she had chosen to keep it to herself and hadn't even told her husband until then as well. Now she won't even talk to us, she sees her parents only at her place (as we live with my in-laws). She has even spoken to a lawyer, I don't think she will take it to court as there hasn't been anymore talk on that, but it was a good possibility. I was worried that my family would be pulled through the courts, my children interrigated and even examined, and even the possibility of my husband facing gaol time, or at least registered as a child sex offender!
Take the advice of smidget, she's been on the receiving end and wished her brother had've spoken to her earlier not 12 years later, she is a very strong woman to even speak to her brother and continue to have a relationship with him now. My sister-in-law may never reach that point, I hope she does as I love her dearly and her children. I'd hate for a young man who clearly loves his siblings not to have a relationship with them and their children in the future because of fear. I honestly believe that the best person to talk to is someone you trust, it sounds like you don't trust your parents to be objective. Seek out another family member, your older brother, an uncle or aunt, or as I've said earlier a church pastor is always a good person to speak to. I find that if you speak to someone at school (I'm a teacher) that they're hands can often be tied and they have to refer something like this on and that's really the last thing you, your family and your sister need. You NEED to talk to someone soon, it's eating you up inside and that is not healthy. I worry for you and that's why I keep checking on this thread. Everyone here seem to feel the same, we want you to move on from this and lead a happy healthy life where you will eventually find a beautiful young woman who'll stand by you and bear your children. If you don't move on from this and put it behind you any relationships you do develop will be affected, it's a given, we all take baggage into relationships and if we haven't made peace with the baggage we have we can not have healthy relationships.