Is it permissible for me to marry my maternal cousin whose younger brother was nursed with my younger brother, given my ignorance as to whether the suckling was sufficient (five or more feedings)?

Lactation establishes the same prohibitions as lineage, for the Prophet, peace and blessings be upon him, said: "What is forbidden by lineage is forbidden by lactation." Thus, the nursed child becomes a child to the woman who provided the milk and her husband. This applies specifically to the nursed child, not to his siblings. The maternal cousin does not become a brother through suckling merely because his brother was nursed by the questioner's mother or another woman, whether there is doubt about the number of feedings or their sufficiency. Prohibition is not established except with five full feedings, according to the preponderant opinion, and doubt about the number does not establish suckling.