Is this normal behaviour for a step-sister?

Its been a year since my Dad remarried after my Mom passed away, I already had a younger sister(now 10) before, but now I have another one(step-sister she's 16.)

We've all gotten together really well, but my new sister seems overly affectionate at times.

she demands I hug her and kiss her everyday (which is normal I guess because my youngest sister asks for the same,) but she also likes to sneak into my bed at nights while I'm asleep, and it's weird to find her hugging me every morning(not that I really mind.)

But last Saturday night, while our parents were out on a date, she kissed me (on the mouth)! we were all alone so I thought it'd be a good idea for us to watch a movie and camp out in the living room(we have a giant plasma TV in the living room)after the movie was almost over and we had all changed into our jammies, she leaned in and kissed me because she "loved me" followed by a giggle, and now my smallest sister wants a kiss too!!! I don't know what to do; I just "said uh huh" and dropped it as if it hadn't happened, now they both keep acting weird around me, it's unnerving.