Under Muslim Law marriage with the foster sister is

Fosterage (rada'a) is one of the absolute bars to marriage in Muslim law: a man may not marry a woman who was suckled by the same woman as he was, so a marriage with a foster sister is void (batil).

An irregular (fasid) marriage arises from temporary bars, such as marrying a fifth wife or marrying without witnesses; a marriage within the prohibited degrees of consanguinity, affinity or fosterage is void from the start.