you can write to us more about your past.....like how old were your sister? and was it just kiss or other stuff? how is your relationship with her now? why do you feel bad? you were just a kid....
how is your life now? you said you have a good life, do you have kids?
You are not alone in this kind of situations....I had a guy friend who told me when he was a kid, he had a friend over for night and he started kissing stuff....then this has carried on for him and my friend was doing those to his own little brother.....now they were grown up and have kids....but he was having that guilt feelings on his shoulder.....they were close family and he was seeing his brother and his family....
Unfortunately, some stuff that are happening in your childhood is not reversible and mostly it's not even your fault....you were a kid....how should you know that.....
Probably, the best way is just luck that bad memory into a small box and leave it there....not try to dig into it....nothing good will come out from it anyway....
take care
Marjan