Odd Sibling Behaviour

I picked up a grade 4 girl for her lesson today.  She has a sister - let's call her Alice - in grade 6, and they don't get on brilliantly.

Girl [surprised]: Alice is actually really nice.
Me [skeptical]: Alice - as in, your sister Alice?
Girl: Yeah, she got out of Japanese to come and see how I was.
Me: Did she?  Your sister came to see you?
Girl: Yeah.  She was seeing if I was okay.  F [in grade 3] hit me at morning tea today, and Alice was checking I was all right.
Me: Wow.  Are you okay?
Girl: Well, I still have a scratch on my face [still visible] and he hit my arm which is still sore-
Me: Which arm?
Girl: My right arm...
Me: Hm... This is your sister we're talking about, yes?
Girl: I know.  Weird, right?  Normally siblings... Well, they fight.  A lot.  They're not nice to each other.