Is it permissible for the middle brother to marry his paternal uncle's daughter, given that he was not suckled with her, and that his older and younger brothers were suckled by their paternal aunt, and his paternal uncle's elder daughter was suckled by his mother?

If the daughter of your middle brother's paternal uncle was nursed by your brother's mother, or your brother was nursed by her mother, or she was nursed from milk resulting from intercourse with your brother's father, or your brother was nursed from milk resulting from intercourse with her father, and the nursing consisted of five known sucklings, then it is not permissible to marry her, as they would be siblings. However, if none of that occurred, there is no objection to his marrying her.