-French co(u)sin, Old French cosin < Latin consōbrīnus cousin (properly, son of one's mother's sister) =con- con- + sōbrīnus second cousin (presumably orig. “pertaining to the sister”) <*swesrīnos=*swesr-, gradational variant of *swesōr (>soror sister) + *-īnos -ine1]...
as shown in Fig.1: in subtype A, the couple of first cousins are the offspring of two sisters; in subtype B, the mother of the male cousin is the sister of the father of the female cousin; subtype C is the inverse of subtype B; finally, in subtype D, the couple of first cousins...