"Whose brothers are these?"
Translation:Чиї це брати?
Чиї це брати? --> Whose brothers are these?
Чиї ці брати? --> Whose are these brothers?
In the first version це is the subject, and in the second version брати is the subject described by ці as by an adjective
Compare with singular case:
Чий це брат? --> Whose brother is this?
Чий цей брат? --> Whose [brother] is this brother?