what kind of exposure does your daughter have to other sorts of kissing in your house? for example, any boyfriends for your teenage daughter? it could be that shes seeing that her sister kisses boys, why cant see, and shes trying it out.
this is a good opportunity to talk to her about relationships. obviously shes getting the idea about boys and girls having relationships. this is a conversation you should have with a calm positive atmosphere, not one that shes in trouble. shes simply trying out what she sees. let her do most of the talking, ask her why shes kissing this boy, and that kind of thing. try to find a book on talking about relationships, and perhaps even sex, with your younger kids. ive heard of a book called 'your body belongs to you' that is directed toward little kids, but might not address the kissing specifically.
also, this is probably normal. kids are well aware that boys and girls are different at this age, and they are also well aware that boys and girls sometimes kiss. its funny because for a certain period of time, kids kissing each other is cute, then all of a sudden its not. just try to remain calm about this, allow her time to think and talk to you. it could take her up to 40 seconds to respond to a question (just developmentally kids think that long!) so give her time, and never make her feel like shes bad for doing it. shes just exploring her world and relationships are a part of that.
good luck. i know i would feel wierd about it too if my son were 6 and kissing. thats why its important to remember to protect our kids from exposure to indecent movies or shows... and even from the interactions of our older children. things like a babysitter having a boyfriend over... you know?
anyway, good luck