你 is in the singular, but you go on to use 大家 meaning everybody. So, you either need to change 你 to 你们 or 大家. Or change 大家 to 你. Also please note, in Chinese adjectives also act as verbs. You do not use 是 before a verb. 你很漂亮。You are pretty. 我很忙。I am busy. 你累吗？Are you tired.

If you're trying to ask, "why are you so pretty? Are you a model?" the more natural way to say it in Chinese is: 你为什么这么漂亮？你是模特儿吗？

你 是 漂亮 <- wrong must say -> 你 漂亮