"Who is she?"
Translation:Vem är hon?
For the same reason you can't say "Who is her?".
Questions in Swedish are most commonly formed by reversing the verb and subject.
For example, you wouldn't answer the question with "Her is Anna".
but aren't swedish and english grammar different? when do you apply english rules and when don't you?
You're right about that. As with any language, the grammar between two languages are different in many cases and should be assumed so unless shown otherwise.
Here, I was just simplifying things, because the reason why it's "Hon" in Swedish is near identical to the reason it's "She" in English. Because "Vem/Who" describes a subject.