Breastfeeding does not affect the collateral relatives of the one who is breastfed
Question

My wife’s sister was breastfed by her paternal aunt, when her paternal aunt had a son. Is this son a brother to my wife through breastfeeding or not?
Praise be to Allah.The impact of breastfeeding on the one who was breastfed applies to the one who was breastfed and his offspring. As for his collateral relatives and direct ancestors, they are not affected by that breastfeeding.
Based on that, the brother of your wife’s sister through breastfeeding is not a brother to your wife, because breastfeeding has no impact on the collateral relatives of the one who was breastfed.
Shaykh Muhammad ibn ‘Uthaymeen (may Allah have mercy on him) said: We have three people: a mother (wet-nurse) who breastfed someone else’s baby; the owner of the milk, who is her husband or master; and the child who is breastfed. Each of the three has direct ancestors, direct descendants and collateral relatives. The direct ancestors of the wet-nurse are her father and grandfathers, and her mother and grandmothers, no matter how far the line of ascent reaches; her direct descendants are her sons and daughters [and grandsons and granddaughters], no matter how far the line of descent reaches; her collateral relatives are her siblings, paternal uncles and maternal uncles. The owner of the milk also has direct ancestors, direct descendants and collateral relatives. The child who is breastfed also has direct ancestors, direct descendants and collateral relatives. Breastfeeding does not affect the collateral relatives or direct ancestors of the one who is breastfed. Rather the one to whom the rulings on breastfeeding apply is the child who is breastfed and his direct descendants only. With regard to the wet-nurse, the rulings on breastfeeding apply to her direct ancestors, her direct descendants and her collateral relatives. With regard to the owner of the milk, the rulings apply to his direct ancestors, direct descendants and collateral relatives. This categorization covers all scenarios and makes it easy for a person to work out where breastfeeding has an impact.
I will give an example: Zayd was breastfed by Hind, and she has a husband whose name is Khaalid. Breastfeeding has an impact on Hind’s mother and grandmothers, and also on her daughters, sisters, paternal aunts and maternal aunts. It also has an impact on the mother and grandmothers of Khaalid, who is the owner of the milk, because they are his direct ancestors, and it has an impact on his daughters, because they are his direct descendants, and it has an impact on his siblings, because they are his collateral relatives. Then there remains the one who was breastfed, namely Zayd. Breastfeeding affects his direct descendants, but it does not affect his direct ancestors or his collateral relatives. Hence the brother of the one who was breastfed may marry the woman who breastfed his brother, because breastfeeding does not affect the collateral relatives of the one who was breastfed. It is also permissible for the father of the one who was breastfed to marry the one who breastfed his son, because breastfeeding does not affect the direct ancestors of the one who was breastfed. But it is not permissible for the son of one who was breastfed to marry the foster sister of the one who was breastfed, because he is one of the direct descendants of the one who was breastfed, and the direct descendants of the one who was breastfed are subject to the rulings on breastfeeding." (Ash-Sharh al-Mumti‘  13/423).
And Allah knows best.