Are the sons of the wife's brother by fosterage, who was suckled by his half-sister more than four times, unlawful to the daughters of this sister, and is it permissible for the husband's daughter, who is my wife, to shake hands with her foster sister's sons and uncover her hair in front of them?

If the wife's father nursed five times within two years from the wife's sister, he becomes her son through suckling, and her children become his siblings through suckling. Consequently, consanguinity (mahramiyya) is established between their children, making the wife's sister's sons her paternal uncles through suckling. It is permissible for the wife to shake hands with them and uncover in front of them. This ruling is based on Allah's saying: (Forbidden to you are your mothers… and your sisters through suckling) and the Prophet's (peace be upon him) saying: (What is forbidden by lineage is also forbidden by suckling).