Henry Earnest Dudeny

P22

The famed puzzler, Henry Earnest Dudeny claimed to have found this four hundred year old epitaph in a cemetery.
Two grand-mothers, with their two grand-daughters;
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.

How might this happen ?