Is it εσείς είστε or εσείς είσαστε?
Both versions are completely correct
I wrote είστε εσείς συγγενείς; which was marked wrong; I've reported it, but all of the questions that involve εσείς take only είστε OR είσαστε, never both. It's difficult to figure out which question wants what form.
Don't you have the είστε εσείς in the wrong order, i.e. shouldn't it be εσείς είστε συγγενείς ?
Is συγγενής/είς a noun or an adjective?
Both! It's a noun here.