12) You didn’t hit your sister. Why would I even accuse you of such a thing? She is crying for some unrelated reason.
11) She hit you first.
10) You were jealous of something she was holding or doing.
9) It was hitting time, and she was there.
8) You were under the impression that nobody was looking.
7) There was a bug on her.
6) You were trying to give her a nice, gentle pat, but got too excited.
5) She was too far away to bite.
4) Hitting is fun.
3) She was in your way.
2) Something upset you several days ago, and you just remembered it now.
1) Because you felt like it.