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impregnate their sisters

Jumped right to incest, did you? The idea is that a man could impregnate his sister's partner, yielding a child highly related to both women. His sister would be the child's bio-aunt. Impregnating your own sister is not a good idea biologically, much less in our social and legal setting. I'm a big fan of genetic diversity. I go to great lengths so my AKC dog does not impregnate his sisters.

Educating anyone benefits everyone.