Who is Hermione's sister?!?!
Welcome to FR...
Don't know, but you call a great quidditch match!
Hermione doesn't have a sister.... or rather, she has never mentioned one and you'd think she would, she mentions her parents sometimes. But she's Muggleborn anyway so her sister probably wouldn't be at Hogwarts.
Though there are the two Creevey brothers, who are supposedly both Muggleborn, so you never know.