So you're about 17 now? Are you still living with your sister/family?

I don't think you intended to hurt her. Nor do I think that anything can be gained by telling her or your parents given that it happened 5 years ago and the behavior stopped as you didn't know it was wrong and you stopped.

I think you should try to work on self forgiveness.

This is a grey area and has the potential to be grossly misconstrued if you discuss this in anything but an anonymous fashion. I'm not trying to scare you, however things like this in the hands of a crusader type can do more damage than good.