What is the ruling regarding a brother-in-law marrying his wife’s sister, given that the mother of this sister breastfed her daughter’s children, and the sister herself breastfed one of her own siblings, and the brother-in-law married the sister whom she did not breastfeed?

If a woman did not suckle from her sister, then there is no harm for her sister's husband's brother to marry her. However, if she suckled from her sister with her brother's milk, then she becomes his paternal aunt through suckling, and it is unlawful for him to marry her. The suckling of her sisters from their sister, or the children of her sister from their grandmother, does not affect the aforementioned marriage.