Is the marriage of my brother's daughter to her mother's paternal cousin valid if it is discovered that the uncle who raised her was a foster brother to the girls' mother, and that the husband was nursed by his grandmother who nursed her uncle?

Lactation that establishes prohibitory kinship is proven by the testimony of one woman, provided that there are five known feedings within the two years of the child's life. This applies even if it occurs years after the woman has given birth, as long as milk is still present. The definition of a single feeding is determined by custom: if the child suckles and then stops of their own accord and returns to feed again, it counts as another feeding. If these conditions are met, your husband becomes a mahram to your aunt and her daughters.