levirate
noun
le·vi·rate ˈle-və-rət 
ˈlē-,
-ˌrāt : the sometimes compulsory marriage of a widow to a brother of her deceased husband
leviratic
ˌle-və-ˈra-tik
adjective ˌlē-
Word History
Etymology
Latin levir husband's brother; akin to Old English tācor husband's brother, Greek daēr
First Known Use
1725, in the meaning defined above