Kids do a lot of weird stuff. I won't go into details, but I'll just say kissing your sister is mild compared to some things my kids did. (Nothing serious, just kid's stuff). Besides it is perfectly acceptable for you to kiss your sister. Not in a sexual way and preferably with her knowledge, but it does not sound like you used her for gratification, just practice. Many families kiss each other on the mouth even after the kids are grown. IMO you need to stop beating yourself up over it. You sound like a good kid to me. Leave the past in the past and look forward to the future!