Foster brothers and sisters are those relatives who are related because they suckled from the same woman.
Prophet
Muhammad was suckled by various women, therefore he had numerous foster brothers and sisters. Their details are given below:
Through Thuwaybah
Hamza ibn Abdul Muttalib . He was a paternal uncle of the Holy
Prophet
but since they had suckled from the same woman, he was his foster brother as well.
Abu Salma ibn Abdul Asad who was the son of Prophet Muhammad’s paternal aunt. Hence, he was his cousin and foster brother.
Masrooh was Thuwaybah’s own son who became a foster brother to Prophet Muhammad . 1
Jaffer ibn Abu Talib . He was his cousin and his foster brother.
Abdullah ibn Jahsh was a paternal cousin and a foster brother of the Holy Prophet . 2
Through Halimah Sadiya
Abdullah ibn Harith was Halimah Sadiya’s own son who became Prophet Muhammad's foster brother.
Aneesa binte Harith and Sheema binte Harith were daughters of Halimah Sadiya who became Prophet Muhammad's foster sisters.
Abu Sufyan ibn Harith ibn Abdul Muttalib who was a distant paternal cousin of the Holy Prophet , became his foster brother as well. 3
Islam
gave these nursing mothers and relations a special status in the family. If a child is reared and breastfed by a woman other than the real mother, she is called Umm-e-Rada'ah (ام رضاعۃ) (foster mother, or milk mother). Her children are treated as real brothers and sisters and marrying with them is prohibited. Thus, a woman who nurses a child, becomes mother by milk-relation. Her suckled child is considered as a full sibling to the foster-mother’s other children, and as mahram to the woman.
Islam
stands unique to accord such a status to the suckling mothers.