The concept of foster relationships, particularly in the context of marriage, is mentioned in the Hadith of the Prophet Muhammad, peace and blessings of Allah be upon him, who stated: "Foster relations are treated like blood relations (in marital affairs)." (Narrated by Al-Bukhari and Muslim) This directive highlights that foster relationships—established through the act of breastfeeding—are considered equivalent to blood relationships when it comes to marriage prohibitions. This means that a person who is a foster sibling, through the act of nursing, holds the same status in terms of marriage restrictions as a blood sibling. Scientific Confirmation of Foster Relationship and Immune Transfer Recent scientific research has provided significant insight into how breastfeeding creates these familial bonds. Specifically, studies have revealed that antibodies and immune factors are transferred from the mother to the infant through breast milk. When an infant is fed on the milk of a foster mother, the milk contains various immune cells and antibodies that help to build the infant's immune system. These antibodies are not only crucial for the infant’s health but also carry certain hereditary characteristics from the foster mother, which the infant absorbs.

Immune Antibodies in Milk: Scientific studies show that the milk of a nursing mother contains antibodies, which can stimulate the formation of immune bodies in the suckling infant. This process happens after multiple feedings—typically ranging from three to five feeds—which are needed to establish a robust immune system. Genetic Influence: The antibodies and other immune cells passed through the milk influence the immune system of the infant, which may share certain hereditary characteristics with the foster mother. This can include immune responses or other genetic traits, making the foster child biologically linked to the foster mother’s family.

One of the key aspects of this scientific discovery is that the immune system transferred through breastfeeding can also carry potential risks if foster siblings intermarry. As foster siblings share immune-related characteristics due to the milk's transfer, intermarriage could cause potential genetic or immune system complications, such as the formation of incompatible immune bodies, leading to allergic reactions or other medical issues. This reinforces the wisdom behind the Islamic prohibition of marriage between foster siblings.

Cellular Transfer through Milk: Mother's milk is composed of not only antibodies but also a variety of cells, including those involved in immune response. This transfer is significant because it links the suckling infant to the mother's hereditary system, mixing cells and genetic factors, potentially contributing to shared familial traits. Immune System Immaturity: An infant’s immune system is not fully developed at birth, which makes it more receptive to foreign genetic material, such as that transferred through the milk. Over time, as the child’s immune system matures, this transfer of immune cells and hereditary characteristics becomes more pronounced, supporting the understanding that the infant absorbs significant traits from the foster mother and her family.

The prohibition of marriage between foster siblings, as outlined in Islamic law, finds its roots in both divine wisdom and scientific understanding. The transfer of immune bodies and genetic factors through breast milk creates a biological and immune connection between foster siblings, which could result in medical complications if they were to marry. This connection between the scientific explanation and the Prophetic directive underscores the profound wisdom of Islamic teachings, which continue to align with modern scientific discoveries. Thus, the prohibition of marriage between foster siblings serves both a spiritual and practical purpose, preserving the health and well-being of individuals and families. As Allah, Most High, has said:

This verse and the teachings of the Prophet emphasize the importance of fostering healthy and responsible relationships, highlighting the unique and protective wisdom in Allah's laws.