How stupid can the sister be to think she’s enough wipe out an army of ants who already ...

Well she's seven

Still not an excuse honestly even at that age you can start to understand right from wrong

Not always. Most kids at that age don't like being told they can't do something because it's dangerous. Even if she knows right from wrong her brain still isn't developed enough to make smart choices based on what older people would think of as common sense.

Exactly what I said she knows but doesn't care

Really? That's not what I read. Maybe that's what you meant, but thats not what you wrote.

Anyway there's a reason why kids aren't criminally liable until they're at least 15. Before that, it's the guardian's responsibility.
Anyway, our brain is not done until we're 27 so let's just say she has some growing up to do.
Not to say it doesn't bother the heck out of me.

Agreed. It's true that the brain doesn't finish developing until mid 20's which is why we still do stupid shit until we get closer to 30. So expecting a child to make rational decisions is like expecting a tiger to not tear your face off if you provoke it. It's just not plausible.