Chopped from Back to the future re "Like kissing your brother":
She says that in the film, too. How does she know?
Maybe it feels like kissing her brother because Marty takes after him, to the point where they have similarly-shaped lips?
Or, you know, it could just be the fact that "like kissing your brother" is a relatively common saying for kissing someone you have no sexual attraction to. It's like when people react to She Is Not My Girlfriend by saying that the not-girlfriend/boyfriend is like a sibling to them and actually mean it.