"Are you her daughter?"
Translation:Czy jesteś jej córką?
What we call a dummy pronoun is the word "to" (and it cases) when it can be translated into English as [this/that/it] (or [these/those]) and mean basically the same thing.
E.g. "To jest kot" ([This/That/It] is a cat) or "Nie lubię tego" (I do not like [this/that/it]).