When is it permissible for an adult to be breastfed in the case of sisters for the purpose of establishing mahramiyyah, and is it permissible for a sister to breastfeed her sister or vice versa to allow the children from both sides to see each other?

Scholars have differed regarding whether the suckling of an adult establishes mahramiyyah (prohibition of marriage) or not, and the preponderant opinion according to us is that it does not. If the woman is a sister by lineage or by suckling, then her children are mahram (definition) to her sister because she is their maternal aunt (khālah). Allah the Almighty mentioned the maternal aunt among those prohibited (for marriage) in His saying: "Prohibited to you are your mothers, your daughters, your sisters, your paternal aunts, and your maternal aunts." He also mentioned the children of the sister among the mahram in His saying: "And let them not expose their adornment except to their husbands, or their fathers, or their husband's fathers, or their sons, or their husband's sons, or their brothers, or their brothers' sons, or their sisters' sons." This also applies if she is a sister by suckling, based on the saying of the Prophet, peace and blessings be upon him: "Indeed, suckling prohibits what lineage prohibits."