Keeping track of family relations can be difficult.
If Edna marries your mother’s uncle Charlie, what should you call her?
This attitude presumably has an evolutionary basis: our genes survived through the ages because our ancestors made efforts to help them survive by caring not only for themselves, but for their close relatives too.
Indeed, there is an ancient Bedouin Arab saying, “I against my brother, my brothers and me against my cousins, then my cousins and I against strangers”, which nicely illustrates the philosophy of caring most for those who are genetically closest to us.
Thus, children of first-cousins are second-cousins, and children of second-cousins are third-cousins, and so on.
In fact, if we regard siblings as 0-level cousins, then this reasoning applies to siblings too: children of 0-level cousins (ie, siblings) are themselves first-cousins.
Thus, first-cousins share two grandparents (but no parents), and second-cousins share two great-grandparents (but no grandparents), and so on.
It follows that if A and B are n-level cousins, then A’s child and B’s child are (n 1)-level cousins.To see where your cousins come from, we have to move up to your parents’ level.Your parents’ siblings are your aunts and uncles, and their children are your first-cousins (since you and they share the same grandparents, but not the same parents): If your cousins have children, then what are they to you?Your great-great-aunt is a sibling of your great-grandmother (n = 3).Your second-cousin-once-removed achieved that designation by being the second cousin (n = 2) of your mother (level m = 1 ancestor), while your third-cousin-once-removed achieved that designation by being the daughter (level m = 1 descendant) of your third cousin (n = 3). One of the reasons we care about family trees is because of a sense that certain family relations are “more related” to us, and should be assisted and protected and loved on that basis.This raises the question of just how similar our relatives’ genes are to our own.