Yours, Etc.: Origins and Uses of 8 Sign-Offs
by 1764, from kin + -ship. A more pure word than relationship, which covers the same sense but is a hybrid.
kinship kin·ship (kĭn'shĭp')n. Connection by blood, marriage, or adoption; family relationship.
A relation between two or more persons that is based on common ancestry (descent) or marriage (affinity).