Is it permissible for a brother by suckling to see the wife of his brother by suckling? And is it permissible for a brother by suckling to see the four wives of his brother by suckling?

The rule is that breastfeeding prohibits what lineage prohibits. Scholars have differed on the prohibition of affinity through breastfeeding, holding two opinions:

First Opinion: It is prohibited. This is the view of the majority of scholars. They cite the generality of Allah's saying: ﴿And do not marry those [women] whom your fathers married﴾ and His saying: ﴿and the wives of your sons who are from your [own] loins﴾. They argue that the word "sons" includes both lineage and breastfeeding, and that the qualification "from your [own] loins" is to exclude adopted sons.

Second Opinion: Breastfeeding does not affect affinity. This is the opinion of Sheikh al-Islam Ibn Taymiyyah and was chosen by Sheikh Ibn Uthaymeen. They cite Allah's saying: ﴿and the wives of your sons who are from your [own] loins﴾, arguing that a son through breastfeeding is not "from the loins." Ibn al-Qayyim refuted the argument of the majority by stating that the word "son," when used unqualified, does not include a son through breastfeeding, so how could it include one when qualified by being a "son from the loins"?

The more cautious approach: Is for each man's wife to observe hijab (definition) from the other, acting upon the opinion of Ibn Taymiyyah. However, regarding what is prohibited in marriage, the opinion of the majority is followed; thus, the wives of a father through breastfeeding are not permissible, nor is the wife of a son permissible for his father through breastfeeding.