What is the ruling on the marriage of the first wife’s daughter to her paternal cousin (the first wife’s son) in the event that the second wife breastfed the first wife’s daughter, her granddaughter, and her grandson, and is the first wife’s daughter considered the paternal aunt of her nephew through رضاعة (radha'a - fosterage)?

What is established by lineage is also established by fosterage, as the Prophet, peace and blessings be upon him, said: "What is forbidden by lineage is forbidden by fosterage."

Thus, whoever is suckled by a woman becomes her son and the son of her husband, the owner of the milk. Therefore, the granddaughter mentioned is considered a paternal aunt by fosterage to her paternal cousin, because she is her father's sister by fosterage. Hence, she is not permissible for him.

However, the fosterage that establishes prohibition must consist of five or more full sucklings.