Potentially the statute of limitations with respect to the alleged matter that you did is time barred under your state's criminal statute of limitations. The problem from your sister's standpoint even if she reported you is that you were a 12 year old minor at the time and having the prosecution prove the elements of the crime after all these years. I suggest that you consult with a criminal defense attorney about the matter you have written about as to what you should or should not do with respect to your sister.