Does the sister of the father's first wife become an adoptive paternal aunt (ʿamma bi al-raḍāʿa) to the children of the son if she is nursed by their grandmother (the second wife of the grandfather, not the father's mother), and does the father become her adoptive brother?

If an infant girl nurses five times from a woman, the nursing woman becomes her mother, and her husband becomes her father through fosterage. Their children become her siblings.

Accordingly, if a girl nurses from her grandfather's wife, her grandfather becomes her father through fosterage, and she becomes a sister to all his children. Thus, she becomes a sister to her own father, meaning she is your paternal aunt through fosterage.

Your uncle, who is your father's brother, is considered a brother to this girl. Therefore, it is not permissible for your cousin to marry her because she is his paternal aunt through fosterage.

If the marriage has taken place, separation between them is obligatory. The children, however, are attributed to their father because they were conceived through a questionable union, and lineage is established through such a union.

The Imams have agreed that if a child nurses five times from a woman within two years, before weaning, he becomes her child. The man whose intercourse caused the milk to flow becomes his father. If the man and woman become the foster parents of the nursing child, then all their children become siblings to the nursing child, whether they are from the father only, or from the woman, or from both, or if they are their children through fosterage.

All relatives of the woman become relatives of the nursing child through fosterage. Her children are his siblings, her grandchildren are the children of his siblings, her parents are his grandparents, and her brothers and sisters are his maternal uncles and aunts. All of these are forbidden for him to marry.