How is she hurting her? Being too rough? At 10 I think she should probably know that it is easy to hurt a younger child. My dd is 5 and occasionally and accidentally hurts her little sister (2) in play, but she knows she shouldn't and I generally remove her from the game for a minute or two, cool down rather than punishment
Yes, being rough I guess. Well I think so, I never see it, it happens when I'm not in the room. Can't tell if it's deliberate or not and whether my expectations are too high that it shouldn't be happening.
I think it shouldn't, personally - with a 5 and 3 year old, yes, I would expect it to happen occasionally, by accident (and the odd lie about that too!). But at 10, I think she should know how to play in a way that doesn't hurt her little sister (the odd accident, yes, that can happen to anyone). What does she say about it when you ask her what happened?