
Yuuchi and Ren's father are half brothers but they can also be cousins genetically. So they share about 25% of dna, their sons will share half of that so to summarize it all, Yuuchi and Ren are distant relatives. There is a possibility that both alphas were to end up with each other, so it's not so incest since they are very very distant relatives.
I love to see their relationship grow even more. Like Souichi finally saying 'I love you' to Morinaga without being force.