What is the ruling regarding the marriage of the eldest brother from the father's side to a foster sister (who was breastfed by the questioner's mother and also breastfed with her by her own mother), and what is the status of their children if the marriage is invalid, given that it was due to ignorance of the rulings on fosterage?

The girl who breastfed from the questioner's mother five known feedings is a sister through rada'a (nursing) to his brother from the father's side. Therefore, their marriage must be annulled immediately. The children are attributed to the husband because it is a marriage of شبهة (doubt/ شبهة of unlawfulness) due to the spouses' ignorance of the prohibition. However, if the nursing was less than five feedings, the marriage is valid.