Animal Family

• Cousin: the most classificatory term; the children of aunts or uncles. One can further distinguish cousins by degrees of collaterality and by generation. Two persons of the same generation who share a grandparent count as "first cousins" (one degree of collaterality); if they share a great-grandparent they count as "second cousins" (two degrees of collaterality) and so on. If two persons share an ancestor, one as a grandchild and the other as a great-grandchild of that individual, then the two descendants class as "first cousins once removed" (removed by one generation); if they shared ancestor figures as the grandparent of one individual and the great-great-grandparent of the other, the individuals class as "first cousins twice removed" (removed by two generations), and so on. Similarly, if they shared ancestor figures as the great-grandparent of one person and the great-great-grandparent of the other, the individuals class as "second cousins once removed". Hence one can refer to a "third cousin once removed upwards."
Cousins of an older generation (in other words, one's parents' first cousins), although technically first cousins once removed, are often classified with "aunts" and "uncles." Similarly, a person may refer to close friends of one's parents as "aunt" or "uncle," or may refer to close friends as "brother" or "sister," using the practice of fictive kinship. English-speakers mark relationships by marriage (except for wife/husband) with the tag "-in-law." The mother and father of one's spouse become one's mother-in-law and father-in-law; the female spouse of one's child becomes one's daughter-in-law and the male spouse of one's child becomes one's son-in-law. The term "sister-in-law" refers to three essentially different relationships, either the wife of one's sibling, or the sister of one's spouse, or, in some uses, the wife of one's spouse's sibling. "Brother-in-law" expresses a similar ambiguity. No special terms exist for the rest of one's spouse's family. The terms "half-brother" and "half-sister" indicate siblings who share only one biological or adoptive parent.