Είστε έτοιμες (feminine plural) / είστε έτοιμοι (masculine plural).
how do I ask the same in you (plural)?
Aye aye, captain!
Am I correct to assume that the person to which this question is addressed is a woman?
A woman or a girl, yes.
I think you'll find looking things like this up in Wiktionary very useful, it usually includes grammatical gender information. :)
Έτοιμη refers to someone in the feminine. Since a girl, κορίτσι, is neuter, why wouldn't it then be έτοιμο?