Levirate

 https://www.google.com/search?q=Levirate

Posthumous

 https://www.google.com/search?q=Posthumous+marriage

humorous

 https://www.google.com/search?q=humorous+marriage

Ghost

 https://www.google.com/search?q=Ghost+marriage

necrogamy

 https://www.google.com/search?q=necrogamy

dead

 https://www.google.com/search?q=dead+marriage