From Longman Dictionary of Contemporary Englishmiddle brother/child/daughter etcmiddle brother/child/daughter etcBETWEENthe brother etc who is between the oldest and the youngest → middle
Examples from the Corpus
middle brother/child/daughter etc• Now, for the middle child.• My older brother played Elvis music at his wedding, and my middle brother and I rolled our eyes.• The wronged self Peter is the middle child of five.• They may send the middle children to Jessy's parents in an attempt to get the eldest through secondary school.• My middle daughter was like that, tall and slim and you could hardly tell.• Richard Nixon: A middle child who became known for diplomacy in foreign affairs, among other things. 