A funny conversation

This is a conversation between a boy and me. We were doing an apple bobbing spot for a Halloween party. 

Walker: Didn't that girl come like five times already?

Me: She has a twin that's also wearing a princess costume.

Walker: Oh.

Later she comes again.

Walker: Do you have a twin?

Little Girl: I think so. Yes I do. 

Walker: I knew it! I knew she had a twin sister.

Me: I told you in the first place.

Walker: Yeah, but you didn't say they were sisters.

Me: 'slap my forehead' Oh my goodness I said the two princesses were twins. What else could they be other than sisters?