What is the ruling on a man marrying his sister's daughter through suckling (the maternal aunt of his children through suckling), and what is the ruling on their children after they become aware of that?

We ask Allah for mercy and forgiveness for your father. If his being a maternal uncle to your mother through suckling is proven, then the marriage is invalid. You and your siblings are attributed to him and inherit from him, based on the statement of Sheikh al-Islam Ibn Taymiyyah: "And whoever marries a woman in an invalid marriage... then his child from her is attributed to him by lineage, and they inherit from each other by the consensus of the Muslims." There is no sin upon you or your parents if they were unaware of the suckling. As for your mother, she does not inherit from her husband due to the invalidity of the marriage. The quorum by which suckling testimony is established is a matter of dispute, and the preponderant view we adopt for fatwa is five satisfying sucklings.