Technically it's not incest anymore since Shisei is half-Russian and Saito is full Japanese. Their DNA is a good bit more distanced thanks to his white Uncle.
Even if true (I don't remember the details of their family tree), that's not how genetic inheritance works. The factor by which an effectively unrelated Japanese man is more closely related to a Japanese person than (for example) a Russian man is essentially negligible in comparison to the strong familial connection between first cousins. Or, to rephrase in a less convoluted way, the genetics of Shisei's father have no effect whatsoever on the fact that she gets fully half her DNA from her mother, who is Saitou's aunt.
Some definitions of "incest" don't even require close blood relations, so even if one of the two is adopted it may not unambiguously rule it out.