Is it permissible for the neighbor's son to marry the daughter of the husband's son if the stepmother breastfed the neighbor's son?

The neighbor's son is not permitted to marry the girl if the second wife breastfed him with milk resulting from sexual intercourse with the girl's grandfather. This is because the grandfather becomes the father of the boy through suckling, and the boy becomes the paternal uncle of the girl through suckling. However, if the milk resulted from sexual intercourse with someone other than the grandfather, then marriage is permissible. The suckling that prohibits marriage is that which occurs within the first two years of life and consists of five distinct sucklings.