*Technically* it’s not incest, at least as our society currently defines that. Incest is a sexual relationship with a *near* blood relative such as sibling, parent, or grandparent. Breeding with cousins isn’t incest ... it’s just icky.
The term the writer should have used was “inbreeding” rather than “incest.” Interestingly, inbreeding is sometimes used by breeders of horses, etc. in search of strengthening a particular physical trait. But of course, horse breeders have the freedom to cull the genetic disasters.
Which is what warfare used to do for them.