What is the ruling on this boy marrying your sister, and what is the ruling on their children, given that the sister was nursed along with a half-brother from the father's wife, and you only learned of this nursing relationship after the marriage and the birth of children?

If it is established that the questioner's sister breastfed her husband, then he must separate from her because she has become his maternal aunt through breastfeeding, according to the Prophet's (peace be upon him) saying: "What is prohibited by lineage is also prohibited by breastfeeding." The wife acquiring the status of a sister through breastfeeding is sufficient to make the questioner a maternal aunt to her husband. There is no sin upon them as long as the matter was unknown, and their children are lawfully attributed to him.