"Where do your sisters go?"
Translation:Dove vanno le tue sorelle?
You're in luck! If you like rules to follow you will love this page; I find it very helpful.
Try to use it again, it should work, but keep in mind that on Duolingo the formal you is indicated by capitalization (Sue sorelle or Lei). Though you'll find this isn't always the case in Italian, we prefer it for a few reasons: this program is based on out of context text and we want to prevent learners from getting confused, most teachers of Italian use this method, and Accademia della Crusca also supports capitalization of the formal you.