First of all, we’d like to clarify that for breastfeeding to have the effect of making a mahram relationship, two conditions must be met: (1) The number of breastfeeding sessions should be five or more, and (2) this should happen within the first two years of the child’s life. If these two conditions are met, then the rulings concerning breastfeeding will apply, i.e. the child will be considered a relative and marriage will be forbidden, etc.
The eminent Muslim scholar and renowned Da`iyah, Sheikh `Abdel Khaliq Hasan Ash-Shareef, states: “The one who breastfeeds a child will be a mahram to the child, if a lady breastfed her sister’s child 5 suckling or more, this will be the Shariah ruling provided this happens when the age of the child is less than two years.”