That's definitely part of it, but I think (and I also remember) that a part of it is that children have a well-developed sense of personal space and who they ar

That's definitely part of it, but I think (and I also remember) that a part of it is that children have a well-developed sense of personal space and who they are close enough to for different types of contact, from a pretty young age, but their sense of closeness is (quite reasonably) derived from actual (remembered) contact rather than from having a genetic relationship or contact before they can remember.