If the kids are not told who their father is because it's deemed unimportant (regardless of whether the mother knows it, so it's not like I'm saying they have sex with anyone), it's definitely possible to have sex with your half-sibling without knowing.
Even if every woman has sex with the same man all of her life, if the men have sex with several women they could easily have children in different households, who might meet once adult and have sex and children.
Even if the kids meet each other's parents, the two women might not know they both had sex with the same man and that he fathered both their children.
I don't see what it has to do with making thoughtful choice. Hell, one could think that if you make thoughtful choices, the same men are likely to be deemed suitable partners by more than one woman.