Answered by Sulma Badrudduja
Question: Would breastfeeding an infant make the two mahram only to each other or to anyone related by blood to the feeder e.g. will the sister of the feeder be considered the child’s mahram or not?
Answer: In the Name of Allah, the Most Merciful and Beneficent
In the Hanafi school, breastfeeding an infant under the age of two lunar years establishes a milk-relationship. The general principle is that the mahram kinships of suckling are the same as the mahram kinships of blood relations. Therefore, the sister of the woman who suckled a male infant, for example, would be the boy’s milk-aunt, and therefore there would be a mahram relationship between the two.
[Al-Lubab fi Sharh al-Kitab]
And Allah knows best.
Checked and Approved by Faraz Rabbani