Or in some situations you need to get lucky and get multiple additional villagers events to help the gene pool.

In a VV1 game I have, I never bought L3 Farming. This means that the village is not "stable" and if I forget about it for too long a time, I can get a near starvation situation with only a couple of villagers (as well as the GC.)

Once I had two males, one kid and one that was "reset" in age, so he didn't starve along with the rest. A little girl showed up and the village repopulated.

Just a few days ago, I had two twin boys and a younger girl (she might have been a sister, but I don't know.) They finally grew up and the girl is busy having babies. I've been mating the girl with only one of the boys, so the uncle could mate with his nieces, but eventually there may need to be sibling matings.

But in most of my games I'll set one male (late) or one female (early) to be a parent, so the chances are that all the kids of the same generation are at least half siblings.