The general way this works is this: First, you need to figure out how the property was titled. When each heir died -- G-ma and Aunt -- who then inherited their share? Did they have a joint tenancy, where the survivor inherited, or did they have something like a tenancy in common, where each person willed their own share to their heirs?
In the JT, the final survivor is the owner -- let's say G-ma died first and Aunt survived. Then when Aunt died her heirs become owner of the property (through probate of the estate). Probating the estate would establish the current owner, and then the current owner could sell to you.
In the TIC, each person's heirs -- G-ma's heirs and Aunt's heirs -- would each own a share, and then probating THOSE two estates would establish the owners of the decedent's share. THOSE people would be the owners now, and could negotiate the sale of the home to you.
If you have been named in the will as the heir, then probate will establish your ownership.
New York, and NYC in particular, have some quirks in property law that are not general to the rest of the USA, so you should absolutely consult an attorney.