The state Court of Appeals recently ruled that it is. The recent decision in Nguyen v. Holder centered on the validity of a marriage of a half-uncle to his half-niece under New York Domestic Relations Law §5(3). According to the Court of Appeals, this type of relationship was not defined as incestuous under the statute. The judges elaborated on this decision through concurring opinions.
Ambiguity in the statute was something that was addressed in the concurring opinions. The Legislature specified, in the statute, that a marriage was void between “[a] brother and sister of either the whole or the half blood.” (Emphasis added) However, in describing marriages between uncles and nieces or aunts and nephews as void, it did not include the same half-blood specification. Judge Smith described this ambiguity in his concurring opinion and stated that, since the Legislature could have added this clarification if it intended to bar all half-blood relationships, the provision must be read as permitting them.
Judge Graffeo’s concurring opinion focused on the role of the Legislature. He stated that if the Legislature wished to prohibit half-blood marriages in the future, it could revisit and revise the statute. There were other concerns that the court was not in the position to address, such as familial structure and the true genetic risk of such a marriage, which would be better dealt with by the Legislature itself. However, some of the judges did express the opinion that marriage between a half-uncle and half-niece was comparable to marriage between first-cousins, which is legal in New York.
You can read a more detailed New York Law Journal article on the subject here: http://www.newyorklawjournal.com/this-weeks-news/id=1202675040046/Court-Sees-No-Bar-to-HalfUncle-HalfNiece-Marriages?mcode=1202615038803&curindex=2&slreturn=20141004130704