adelphogamy

But she also asked about the civic organization of our state : whether our queens were adopted or whether we insured the continuity of our dynasty by adelphogamy.

1947, Frederick Philip Grove, Consider Her Ways, page 142

[…] at other times, and indeed rather frequently, the union of brother and sister, or adelphogamy, as the entomologists call it, is accomplished in the nest itself; for our humble heroines find it easier than we do to modify, in case of need, their fundamental laws, or even to reverse them, adapting themselves to circumstances, and turning these to account.

1958, Maurice Maeterlinck, The Life of the Ant, page 56

Concerning the system of polyandry among the Todas, I gather that a woman may be married to more than one man, provided they are brothers (adelphogamy), and the maximum number of husbands seems to be five.

1894, Edgar Thurston, “Anthropology: Todas of the Nilgiris”, in Madras Government Museum Bulletin, volume 4, number 1, page 6

While there are obscure (and presumptively vestigial) traces of polyandry or adelphogamy, and while an informal polygyny is practiced by the chiefs and older warriors, the formal matings are between one man and one woman, and appear to be permanent.

1898, W. J. McGee, The Seri Indians, page 285