If the baby
was breastfed at least five times during the first two years of his life by (Part No. 21; Page No. 107) the first wife, her husband becomes a milk-father to him. Therefore, the baby becomes a Mahram to the second wife, because he is her husband’s milk-son. Allah (Exalted be He) says:
...and not to show off their adornment except only that which is apparent (like both eyes for necessity to see the way, or outer palms of hands or one eye or dress like veil, gloves, head-cover, apron, etc.), and to draw their veils all over Juyûbihinna (i.e. their bodies, faces, necks and bosoms) and not to reveal their adornment except to their husbands
It is also authentically reported from the Prophet (peace be upon him) that he said,
“Breastfeeding prohibits what birth (consanguinity) prohibits.”
It is authentically reported from
`Aishah (may Allah be pleased with her) that she said: "Amongst what was revealed from the Qur’an was: 'Ten known breastfeeding make (marriage) Haram (prohibited)...' It was then abrogated by: 'Five known breastfeedings...' The Messenger of Allah (peace be upon him) died while such was the practice." If the baby takes a breast, sucks milk from it, and leaves it to breathe or move, this is considered one breastfeeding and if it does it again, it is considered as a second breastfeeding, and so on.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.