Which brother to be brought back to life?
Yudhi asked for sahdev.
Why?
Because all sons of pandu come from two mums so, if two sons can live then both have to be from two mothers to respect and continue their line and sahdev was the youngest of madri too.
Gandharv was so impressed by the answer that he brought all the four back to life.
Yudhi could have asked for arjun or bheem since they were of kunti but he was above that.