[Ruling for One Who Learned Later of a Breastfeeding Tie with His Wife]

Answer—and Allah grants success: The fatwa he received suffices, for this topic of breastfeeding is a matter of ijtihād and disagreement. The nursing described in the question is disputed from two angles:
First: “Milk of the sire” (laban al-faḥl). Some held that the nursling is the child of the nursing woman only, not of her husband; accordingly, the nursling would be lawful to the children of the husband and to his brother.
Second: counting the number of sucklings. Some scholars required five distinct sucklings; others did not consider a specific number, but made the ruling turn on the occurrence of breastfeeding in general.
Yes: if the questioner’s heart is at ease with such a fatwa, we deem it sufficient. If he is not at ease but remains anxious and perplexed, then let him separate from his wife and divorce her so she becomes lawful to other husbands with certainty. Separation is more precautionary, for in the hadith: “Leave that which causes you doubt for that which does not.” And the Most High said: “So give good tidings to My servants—those who listen to the word, then follow the best of it. Those are the ones whom Allah has guided, and those are the people of understanding.” [Al-Zumar:17–18]
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.1