The younger son is unaffected by the breastfeeding
Question: Twenty-nine years ago I nursed my youngest brother. My mother has breast-fed my eldest daughter and my brother’s eldest son several times. My younger son now wishes to marry my brother’s daughter, and that son of hers did not nurse from his grandmother. Is such a marriage permissible or not?
Answer: There is no objection, since the boy in question did not nurse from his grandmother—on the condition that the girl has not nursed from his mother, that he has not nursed from her grandmother, and that she likewise has not nursed from his mother. If these conditions are met, nothing stands in the way of their marriage.
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Marriage to the sister of one’s milk-brother
Question: I would like to marry a girl, but my brother was breast-fed together with her sister. Is it permissible for me to marry her?
Answer: There is no impediment to your marrying your milk-brother’s sister so long as you yourself have not nursed from her mother; she is not forbidden to you even though she is a milk-sister to your brother. Provided that you did not nurse from her mother and she did not nurse from yours, it is lawful for you to marry her—or any of her sisters.
The prohibition applies exclusively to your brother: every daughter of his wet-nurse is forbidden to him. As for you, the breastfeeding relationship does not involve you at all.