"I do not know any of her sisters."
Translation:Non conosco nessuna delle sue sorelle.
I used qualsiasi instead of nessuna, as that was written in the drop down hints, but got marked wrong. Why?
You need two negatives ("non" and "nessuna") when dealing with these kind of sentences.
Here the stress is on that i don't know ANY of the two girls. This is expressed with double negation in Italina - nessuna. Using "alcuna" (though gramatically may be correct) would leave the possibility of knowing one of them.