What is the ruling on the marriage of a son to his mother's full sister through fosterage, given that the mother was fully breastfed by the sister's mother, while the questioner was not breastfed by the sister's mother? And does the questioner become a brother to all of her siblings, or only to her sister?

Fosterage establishes the same prohibitions as lineage. Thus, a child who is nursed by a woman becomes her child and the child of her husband, the owner of the milk. This applies only to the nursed child, not their siblings. Accordingly, the girl who was nursed by your mother is your foster-sister, and your foster-sister alone, to the exclusion of her own sisters. It is permissible for your son to marry her sister, unless there is another reason for prohibition. A foster-sister is not described as a full sister. The prevailing opinion stipulates that for fosterage to be prohibitive, there must be five full sucklings.