Question?

Maybe she was just jealous, because of all the stickers he got ("Your brother has collected the most stickers. So he won again...).

No, seriously: Maybe it were some suppressed feelings of hate, jealousy, sexual tension (maybe a mix of all of the above) that she just had to unload.

No matter how much the parents controlled their kids, they could not erase their psychological and sexual drives (see the brother). They had to "go wild" once in a while.

Cheers


It's too bad she won't live! But then again, who does?
- Gaff (Blade Runner)

Forget the hammer. Why did she slash him with a knife? I think that the point of her repeated violence is that her parents' behavior caused her to lash out in violent ways. The children all react differently to their imprisonment. The son is content, but obviously emotionally unhealthy and childlike in his emotions. The older daughter is rebellious and escapes. The younger daughter picks up on the emotional and physical violence of her parents and repeats that behavior the way that many victims of abuse become abusers themselves.

The youngest was obsessed with administering medical treatment; I think she just wanted an excuse to operate.

The parents upbringing of the children was so twisted, the younger girl could not understand real consequences of real actions. The irony was that this created a menace inside their sheltered compound.