Re: I will never forgive myself.
When you were young , it happened to you... You were still young when what you did with your sister. I am certain this would not have happened if it did not happen to you first.
If you could talk to your sister about this, and let her know how sorry your are, and you picked up this behacior because it DID happen to you first. Communication is the key , so you can forgive yourself. Just to let you know it is not unsusal for children to experiment with their siblings or cousins> All you knew was that it felt good , If you knew it was wrong at that age and did it anyway then that is a diffeent story.
Now that you are older and know better, you can see how your behavior may have hurt your sister unknowingly. If your sister ever told you to STOP, or try to get away from you , and u did it anyway to her , then it changes things. If your sisterwas a willing participant , then this is more like child exploration then anything major. Most kids will and have done the excat same thing.
If your unable to talk to your sister, you would feel a lot better if you could go into therapy and work it out in a safe place.
You really need to forgive yourself, seriously, like i said before kids do and will explore each other's bodies... That is not abnormal behavior. But , when a child grows up and then knows it is not right , the behavior stops, but the guilt cdan linger. That is what I think is happening to you.