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Comment by davnicwil | Hacker News Reader

Actually that's often but not always true -

    brother b1 and sister s1 from family 1, with parents g1
    brother b2 and sister s2 from family 2, with parents g2
    s1 has child x with b2 - x has grandparents g1, g2
    s2 has child y with b1 - y has grandparents g1, g2

x and y are cousins, and also share the same 2 pairs of grandparents :-)

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It can get even more fun!

If instead of two brothers marrying two sisters, you have two brothers marrying two first cousins, like so...

    b1 and b2 are brothers
    c1 and c2 are first cousins
    b1 and c1 marry and have child x
    b2 and c2 marry and have child y

... then x and y are both first and second cousins (and their children are second + third cousins, etc).

Those are "double first cousins". I even have some triple cousins in my tree, where three siblings from one family married three siblings from another family.

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