They're beast people. The earlier generations were aiming to increase their population as much as they could since they were minorities and believed in preserving their genes and passing it down. The Top (black haired father of the 3 babies) from the main couple has countless half-siblings from both parents' side, so does his other half-siblings like Kazuhisa's father had been mentioned to have lots of children from different women including Kazuhisa & his half brother (the ML)'s mom. ML's father gave up on having a love life when he was young to sire countless offsprings. Their "family tree" probably looks like a "Spider Web" since everyone is connected through their parents and partners/ex-partners since Beast people are known to mate with a lot of different people.
The ML shares the same father with the older sister whose son plays with his baby and shares the same mom with Kazuhisa (partner of Alex, the dark skinned guy with the deadlock) as I mentioned. They try to maintain good relationship with their half-siblings and half-siblings of half-siblings (As we've seen Kazuhisa knows ML's elder half-sister) and also their parents who might've even been the ex-partner of an ex-partner of their own biological parents! They believe maintaining good relationship and being well connected to everyone gives them the strength they need to survive and not be oppressed or discriminated by regular humans who are the majority and often look down on beast people.

ain't those kids cousins?