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It is in the torah and the talmud that it's a good thing, dare I say a mitzvah, for a man to marry his sibling's daughter.
I know whereof I speak.
I don't know how that worked out for most of the oh, 10's of thousands? of people who did this, but for the family w/whom I'm familiar, it was lethal idiocy.
I've spoken to some rabbis and all confirm this practice.
The couple who did this in this family lived in NYC and had to go to RI to get married, cause of course in RI, that bastion of religious freedom where the first synagogue opened on this side of the pond, it was legal for jewish men to marry their nieces. These days, equity will out, any man can marry his niece in Rhode Island.