Well it sounds like you did molest your very young sister when you were 12-13 and I am sorry but at that age I knew such things were inappropriate. Because of your age, you are partially less responsible than a full adult, but you still should be held partially accountable in my opinion.
However people can change, and if you are truly, truly sorry, and have not repeated this behavior with others, and now would NEVER sexually touch an underage girl, then it is possible you have changed and are now a better person. Only you can know if you still have the potential attraction towards children, and the potential ability to act predatory towards them.
If you are truly sorry, and truly no longer a risk to children or women, and accept responsibility for your actions, then I wish you the best. However it was your young age that protects you, if you were an adult that did this I would think you deserved a long prison sentence and were a bad person.