It seems to be a nazified reading of "inclusive fitness" which is a refinement of Darwin's original idea but extended slightly to groups of individuals

https://en.wikipedia.org/wiki/Inclusive_fitness

I read somewhere that human genetic evolution essentially stopped once the bicycle enabled people to cycle over to the next village to have sex instead of having to bonk their closest relatives, so I don't really see the point from an evolutionary point of view to enforce biological kinship by divine fiat, unless you're unhealthily obsessed with "the purity of the blood". Also, the obsession with only allowing sex for procreation is weirdly reactionary and goes directly against other evopsych fetishes like "alphas" impregnating more females compared to "betas".

Edit Genghis Khan is mentioned in the comments, as someone who has maximized IGF but hardly monogamously.