adelphogamy
/ˌadɛlˈfɒɡəmi/noun
1 Anthropology. Marriage between brothers and sisters, especially between several brothers of one family and a woman, or several sisters, of another.
2 Zoology. Mating between male and female siblings (especially in ants and certain other invertebrates).
Origin
Late 19th century. From ancient Greek ἀδελϕός brother + -gamy, probably after French adelphogamie.