Is it permissible for a daughter to marry the husband of her mother's foster sister, and what is the ruling on such a marriage?

It is not permissible for a man to marry his sister's daughter through suckling (رضاعة), because she is considered among the women prohibited for marriage, similar to a sister's daughter by blood relation (نسب). For suckling prohibits what is prohibited by blood relation, provided its conditions are met. The prohibited women are mentioned in the noble verse: "Prohibited to you are your mothers, your daughters, your sisters, your paternal aunts, your maternal aunts, the daughters of your brother, the daughters of your sister, your foster-mothers who suckled you, and your sisters through suckling..." (An-Nisa: 23).