I'm afraid I might have abused my younger sister when I was 14
Why do you think that you abused your younger sister?
She used to scream when I would pee and then I would take the piss out of her in retaliation.
This sounds like an odd situation and I think you should talk to your parents about this. It is not good to be abusive or cruel to a sibling.
Maybe. I might have forgotten about this since I was so young back then.
If you feel you really did these things I think talking to your sister might help you work through your problems. An apology can go a long way even if it happened long ago.