Sounds pretty natural to me. If I was asking a boy and girl sitting next to each other if they were related, "are you siblings" is probably exactly how I would phrase it.
The English sentence doesn't seem natural to me either. As the Hindi sentence is asking one person (based on the तुम and हो) in English I would say it's asking, "Do you have any brothers or sisters?" or "Do you have any siblings?". If this had been asking two people it seems like it'd be, "Are you brother and sister?"
I'm confused -- is तुम plural as well? In other words, can "tum" be used for "you", when "you" is more than one person?
Yes, but it's usually followed by लोग as in तुम लोग when used as plural, I think. As in you lot, y'all, or you people. English has the same "problem". It's pretty darn confusing for other people.