The word triù-saptabhiù means seven multiplied by three. In one’s family one can count back four or five generations—to one’s great-grandfather or even one’s great-grandfather’s father—but since the Lord mentions twenty-one forefathers, this indicates that the benediction expands to other families also. Before the present family in which one has taken birth, one must have been born in other families. Thus when a Vaiñëava takes birth in a family, by the grace of the Lord he purifies not only that family but also the families of his previous births.

SB 7.10.18

The family in which a mahä-bhägavata takes his birth is fortunate because due to the birth of a first-grade devotee the members of the family, past, present and future up to one hundred generations, become liberated by the grace of the Lord, out of respect for His beloved devotee. Therefore, the highest benefit is done to one’s family simply by becoming an unalloyed devotee of the Lord.

SB 1.12.17