Is it legal in New York for a man to marry his widow's sister ?
It doesn't matter, he's dead
Why this works
This riddle plays with the concept of a "widow." A widow is a woman whose spouse has died, so if a man has a widow, it means he is no longer alive. Therefore, he cannot marry anyone, let alone his widow's sister. The humor and trick lie in the unexpected twist that reveals the man's death makes the question moot.