The point is that this wouldn't start or end with one nuclear family merging. It would keep splitting because you're no longer looking at a nuclear family. You're looking at an extended family who would progressively branch out. So, yes, maybe your brother is your first cousin but what if your second cousin gets on with a first generation? We would have to be assuming that only one subset of family members are reproducing with others of that one subset like cousins for example but it would be more complex than that because a second cousin (the child of two first cousins) could procreate with a first aunt or an original settler. Do you see what I mean? You can inbreed somewhat safely and that's why Papua new guinea has stable tribes. What if we consider a possible Hills Have Eyes/Roanoke theory? What if, instead of eating/cannibalizing hikers, we bring them into the family so to speak? Elderly, not so much, but children? Thats new breeding stock.