But you're living with a group of people to whom you're likely either related directly or have known your whole life (or their whole life, in the case of kids).

But you're living with a group of people to whom you're likely either related directly or have known your whole life (or their whole life, in the case of kids). You're applying your numerate bias here. I have no idea if that's how the Pirāha handle the situation, but it's certainly plausible. Perhaps their memory for this kind of thing is better than yours because they've relied on it for cases where you'd just keep track of a number.