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Yes, this marriage should be void because of the degree of kinship. The law provides as follows:
Domestic Relations Law 5
Incestuous and void marriages
A marriage is incestuous and void whether the relatives are legitimate or illegitimate between either:
1. An ancestor and a descendant;
2. A brother and sister of either the whole or the half blood;
3. An uncle and niece or an aunt and nephew.
If a marriage prohibited by the foregoing provisions of this section be solemnized it shall be void, and the parties thereto shall each be fined not less than fifty nor more than one hundred dollars and may, in the discretion of the court in addition to said fine, be imprisoned for a term not exceeding six months. Any person who shall knowingly and wilfully solemnize such marriage, or procure or aid in the solemnization of the same, shall be deemed guilty of a misdemeanor and shall be fined or imprisoned in like manner.
Here's a link to the statute: http://www.jdbar.com/Statutes/DRL-5/drl-5.html
Obviously, this statute was written prior to the advent of gay marriage......but the same principles would apply.