Brothers and sisters, or boyfriends and girlfriends?

Conversation with Kaylin in the car last night:

Me: Cody and Becca are having a baby boy! Peyton is going to have a baby brother.
Kaylin: Ooooh cool!
Me: Yep.
Kaylin: Maybe they could be boyfriend and girlfriend!
Me: No, Kaylin, that's not really how it works. They will be brother and sister, so they won't be boyfriend and girlfriend.
Kaylin: Well, they could. That would be neat.
Me: No, taht really wouldn't be neat. Brothers and sisters are not supposed to be boyfriends and girlfriends.
Kaylin: Who says?
Me: Wellllll....... God does.
Kaylin: Ok. But it would still be cool if they could be boyfriend and girlfriend.

Interesting. Good thing she doesn't have a brother! :)