Those one belongs with, by blood or by choosing. In this house kin is a standing, not a species.
Old English cynn (family, race, kind), from Proto-Germanic kunja, from a Proto-Indo-European root meaning to give birth or beget; those one belongs with by shared origin, or by choosing.
A standing rather than a species — kinship here is claimed by belonging, and belonging can be chosen as readily as it can be born into.