The Grandmothers

Two grandmothers, with their two granddaughters,

Two husbands, with their two wives,

Two fathers, with their two daughters,

Two mothers, with their two sons,

Two maidens, with their two mothers,

Two sisters, with their two brothers,

Yet only six in all lie buried here,

All born legitimate, from incest clear.

 Explain how this may be.

answer:

Two widows each had a son, and each widow married the son of the other and then each had a daughter.