Cooperation is rational in that it can be vastly preferable to non-cooperation or conflict. But it also requires altruism because most preferable of all is to defect while OTHERS cooperate with you. And foregoing that temptation (on behalf of others, more than yourself) is a price that one must pay in order to cooperate.
Cooperation is self-enforcing among kin. And defection is self-defeating among kin. Kinship makes altruism reciprocal because genes which code for kinship altruism help other instances of themselves, and therefore spread and outcompete genes which code for, or don’t code against, defection against kin (which parasitize other instances of themselves.)
Cooperation between non-kin is possible but it is more difficult and costly, it requires more technology: reputation, active enforcement, full accounting, quid pro quo, exchange, warranty, adjudication, punitive measures, etc…
Cooperation between non-kin is therefore more technical than between kin and would best be left to specialists while most people live most of their lives, and do most of their business, among kin – to minimize costs and maximize benefits.