I agree with Nicole Haymans and want to add my own experience.  My paternal grandfather married his half-niece, my paternal grandmother, in 1919 in Providence, Rhode Island.  My paternal grandfather's half-sister was my paternal grandmother's mother.  They married in Rhode Island because their marriage was legal there since it was allowed under Jewish law, but illegal in New York City where they resided.  Peculiarly, under Jewish law a man is allowed to marry his niece but is not allowed to marry his aunt.