She should look up the building on
PropertyShark - Real Estate Maps, Foreclosures, Property Reports and Comparables to see what the other recent sales were (shows back to 2004). Also if you use the map function on that site it is possible to identify other coop buildings in the area (as opposed to rental bldgs and stores or churches) and check the coop sales prices in nearby bldgs as well.
I am not sure what coop organized means. It may simply mean it's a coop and not a condo. Does your sister understand what a coop really is? All the fine details about underlying mortgages and coop boards and subletting rules? If not, she should buy and read the NYC Coop Bible.
A good real estate lawyer is a necessity. The lawyer can read the financials of the building (a coop is a corporation and if the financials are not in good shape then you don't want to buy into the bldg). She doesn't need a lawyer if she's only just looking now, but she should get recommendations for lawyers and have one lined up in case she decides to put in an offer and the offer is accepted.