Why suckling prohibits that which giving birth prohibits
21-11-2006 | IslamWeb
Question:
Asslamualaikum, Can you tell me the reasoning behind why a child who has a wetnurse, is considered a sibling of any children sharing that womans milk. Wassalaam
Answer:
All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
The main reason why it is forbidden for a person to marry someone who breastfed from the same woman as he/she did, is that this is the ruling of Allaah, His Messenger 
. This ruling is proved by the texts of the Quran and by the Sunnah of the Prophet . Allaah Says (what means): {Prohibited to you [for marriage] are your mothers, your daughters, your sisters, your father's sisters, your mother's sisters, your brother's daughters, your sister's daughters, your [milk] mothers who nursed you, and your sisters through suckling…}[Quran 4:23]. The Prophet said: "Suckling relationship prohibits that which giving birth prohibits." [Al-Bukhari and Muslim]
Among the wisdoms of being prohibited from marrying due to a suckling relationship is the statement of the prominent scholar Ahmad Ad-Dahlawi 
as he said: ‘The woman who breastfeeds a child resembles the mother, as she contributes to the growing of his body, whereas, the mother carried him [the child] throughout his developmental stages of his creation in her womb, and the breastfeeding woman fed him and fulfilled his need in his first stages of his development after birth, so she is a second mother after the real mother.’
Furthermore, Shaykh 'Abdul-Kareem Zaydaam said: 'Since breastfeeding fulfils some of the material needs between the breastfed child and his breastfeeding mother in the same way as it fulfils one of the major aspects of motherhood and affection, because she hugs and cuddles the child and breastfeeds him from her breasts, then she deserves to be a mother for him due to this breastfeeding and due to the aspects of motherhood and affection that are achieved therein, …Additionally, since the mother becomes forbidden for her child to marry her, then this breastfeeding mother becomes forbidden for her breastfed child, in the same manner his real mother is forbidden for him. Similarly, the same ruling applies to all other members who are related to the mother through suckling exactly as it applies to the mother.’ Those referred to by "all other members who are related to the mother" are the children who suckled from the same mother.
Allaah Knows best.