You start out with only four breedable villagers; and only one's a woman. If you don't have kids right away, you only have two mothers.

The first generation will have to breed between two families; they'll quickly become two branches of one family. Preventing inbreeding is nigh-impossible, even if you don't count cousins, requiring careful charting and record-keeping of familial ties.

Preventing children from going after their parents is all I would ever hope for, especially with them going off to get busy while the game is off; and since I tend to breed my villagers for a certain hair type, (different for each tribe,) I tend to ignore the issue altogether.