Did you receive a copy of the lease and a Tenant Estoppel Certificate related to this tenant? These documents should have been provided during escrow. Your claims and remedies depend upon the kind of lease, duration of lease, notice reqirements and many other factors, in addition to an potential fraud claims. You could have several varying claims, with different remedies, in this scenario.
New York is fairly tough on these types of claims and you must review your contract carefully. However, if the tenant was in fact related and the tenant did make an affirmative statement that was intentionally misleading, you may have a case. Tough one, but a case.
This is not legal advice which would depend on a thorough reviews of documents and the law.