Mother’s milk (own mother or foster mother) transfers genetic material to the child. The Quran and The Sunnah have prohibited Muslims from marrying their foster mothers and foster siblings for a wisdom that Allah knows best.
Allah says:
“Prohibited to you (for marriage) are … foster-mothers (who suckled you) and foster-sisters” (Qur’an 4:23).
Thus, if you were breastfed by a lady, she becomes your foster-mother with five certain breastfeedings (See Sahih Muslim 1452), then you became a foster-child to her, and her children are your foster-siblings (and Mahram), both the older and younger ones among them.
Additional Note: What should be out attitude regarding topics like ‘The Quran and Modern Science’?
1. It is not appropriate to claim that everything from Islamic texts (The Qur’an & The Sunnah) is absolutely compatible to modern science. Because ‘modern science’ is not a fixed state, it keeps changing and shifting. What is ‘true’ now may be called ‘false’ later.
2. Hence we believe that everything mentioned in the Islamic texts is cent percent true because it came down from Allah and was spoken by His Messenger Muhammad ﷺ, regardless of whether it is in accordance with ‘modern science’ or not.
3. At times we may find certain scientific researches to support what’s mentioned in The Quran and/or The Sunnah. We may share it in order to show that what’s mentioned in the Islamic texts is in accordance with nature.
4. However if the scientific research or researchers change their stance, we shouldn’t reject the meaning of The Ayaat of The Qur’an or any authentic Ahadith. Our belief upon them is based on our Imaan in Allah and His Messenger, and that includes belief in The Unseen, Al-Ghaib. Not upon ‘modern science’.
Saheehus-Seerah 