Is the marriage of the youngest sister to her paternal male cousin Islamically valid, given that the mother breastfed the paternal male cousins five full, satisfying times, and the youngest sister was not breastfed with them, and that the sister was engaged when the paternal male cousin proposed to her?

It is forbidden for a Muslim to propose to a woman already engaged to another, unless the first suitor relinquishes his claim or gives permission. What the father and paternal cousin did is against Islamic law. If the male paternal cousins were suckled by the questioner's mother five full, satisfying times, they become her brothers and her sisters' brothers through fosterage. It is then forbidden for them to marry her sisters, based on the Almighty's saying: "and your foster-sisters," and the Prophet's (peace be upon him) saying: "What is forbidden by lineage is also forbidden by fosterage." The paternal cousin and the questioner's sister must be separated. If they refuse, the matter should be referred to the Sharia (definition) court.