It's not correct. The correct sentence should be either "you haven't seen anything yet" or "you have seen nothing yet"

It contains a double negative which is not appropriate usage of English, but it is very common. You haven't seen anything yet. Or. You have seen nothing yet. Or. You've seen nothing yet. All are correct

No, it is not.

No, it is not because it has a double negative. You can say " You haven't seen anything yet!" or " You have seen nothing yet!".

It's grammatically wrong in two different ways: Proper English - the double negative gives a positive meaning. Literally, you are saying "You have still seen something!" Your correct sentence should be "You haven't seen anything yet." Note how the negative is placed as early as possible. Slang English - the pedantic use of a clean present perfect is inappropriate if you're aiming for a slang sentence (yes, slang also follows its own grammar rules!). It should be written, "You ain't seen nothing yet!"