I'm a former realtor and the best method is to go thru NYC Housing to put your sister's name on the waiting list for subsidized housing (NYC housing).
Unfortunately even though realtors can assist you with Section 8, the truth is they don't get paid much for it and to realtors it's a waste of time helping you find an apartment when they are practically working for free. Realtors can make more money helping renter's find regular market rate apartments. Realtors work on commission and don't get paid until you sign a lease, so why spend time with Section 8 if their commission check is so little.
So start contacting NYC Housing.