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.
Dr. Muhammad As-Sayyed Ad-Disooqi, professor of Fiqh (Jurisprudence) at Qatar University, issued the following: “In order that the fostered children become mahrams (where males become unmarriageable to females), it should be when they are under two years old. That is, breastfeeding children above two years of age does not imply being unmarriageable to one another (whether the milk is suckled or given in a cup). For example, when a woman breast-feeds a three or four year old boy – who is allowed to marry her daughter when he comes of age – breastfeeding him does not entail that the boy will be unmarriageable for the daughter. She must breastfeed him at a time when babies nurse, i.e. within the first two years.
This means that breastfeeding her nephews and nieces when they are eight years old does not mean they will become brothers and sisters through the mother’s milk, nor that they become sons and daughters.”