Well I was 10 years old when I had my first boyfriend(who was 9) He wasn't the first boy I had kissed though. I may have been like 5 when that happened. BUT with my first boyfriend we kissed all the time. We never went as far as to actually touching each other I don't think. BUT we were boyfriend and girlfriend for like 8 months. I know WEIRD. LOL I would say the kissing is normal, the other stuff not so much. She could be seeing things on TV.
Please, Do not encourage your daughter to kiss a sibling. Perhaps you should try teaching her the difference between two people deciding to kiss, and one person convincing another to kiss them (amongst other things) and why the first is a better, more rewarding option for both parties involved.
I'm not saying she meant to hurt anyone, I just think she could use some information.