"Whose is this dress?"
Translation:Ποιανής είναι αυτό το φόρεμα;
To answer my own question, here is a good discussion about it. https://www.duolingo.com/comment/17572443
"Ποιανού (=whose) 1) refers to a masculine owner (Ποιανού είναι το βιβλίο; -Του Γιάννη) 2) used when you don't know the owner's gender (Ποιανού είναι το σπίτι; -Της μητέρας μου. -Του πατέρα μου.) Ποιανής (=whose) refers to a feminine owner, when all possible owners are feminine (Κορίτσια, ποιανής είναι αυτό το σουτιέν; -Της Χρύσας)"
It's not rare to use ποιανών, but in this sentence we are talking about one dress, so it's highly unlikely that it belongs to more than one person (at least that's how language usage sees it - it's unlikely that someone will say ποιανών when taking about one dress). In a sentence like "whose are those dresses?", "ποιανών είναι αυτά τα φορέματα " is fine and used. :)