"You are his daughters."
Translation:Vous êtes ses filles.
Daughters is plural. So there are more than one girl. It can't be "tu", but it has to be "vous"
It may help to imagine this scenario:
Fool: "Hello Goneril, Regan and Cordelia"
Regan: "What do you want, Fool?"
Fool: "To ask you to help your King"
Goneril: "We will not"
Fool: "But, you are his daughters!"
Easier to use as a phrase but it can be a sentence on its own:
"The women of the world are the children of God. You are his daughters. Go forth and multiply to honour your God."
Votre describes a noun and means your. For example votre chien means your dog. Votre sont ses filles would then translate to "your are his daughters" which does not make sense.