It is stated that the foster relations are treated like blood relations in the following hadith.
Narated By Amra bint 'Abdur-Rahman : That 'Aisha the wife of the Prophet told her uncle that once, while the Prophet was in her house, she heard a man asking Hafsa's permission to enter her house. 'Aisha said, "I said, 'O Allah's Apostle! I think the man is Hafsa's foster uncle.' " 'Aisha added, "O Allah's Apostle! There is a man asking the permission to enter your house." Allah's Apostle replied, "I think the man is Hafsa's foster uncle." 'Aisha said, "If so-and-so were living (i.e. her foster uncle) would he be allowed to visit me?" Allah's Apostle said, "Yes, he would, as the foster relations are treated like blood relations (in marital affairs)." - Sahih Bukhari.
Is it permitted for a man to marry the foster sister of his brother?
Since, that woman is only his brother's foster sister, not for him.
ie, What about marriage with blood relation's foster relation?