A historical factor is that nearby peoples from the local group (Cave #217) often had relatives in them and the blood ties would have a whole lot more impact on how people behaved
yes, the whole inter-band rivalry modeling thing has yet to be settled. it, raiding, resource depletion, mating, blood ties, captured prisoners, etc, are all tied up together.
at the low level, modeling mating and blood ties is required. as well as taking captives. raiding based on nearby hostile bands and resource depletion based on nearby bands (hostile or friendly) are already modeled. however, modeling mating is not required to implement capturing prisoners. so a basic implementation of captives that doesn't model the blood ties thing could be done. right now things in the game are very all or nothing, badguys shoot on sight. it would be more realistic to have rival bands that could become enemies or friends. like relations for bands, in addition to relations between individual PC's and NPC's. sounds like a job for a politics engine to me. each band would be a faction.