Answer:
Andrew
Andrew's the one sibling that Antonio actually forms some sort of relationship with. When the three brothers return after the war, Andrew is the only one who sticks around for any length of time, and he and Antonio spend time talking on the way to school in the mornings.
But if you're thinking they've got this cool brotherly love thing going on, well, you're going to be sorely disappointed. Much to Antonio's horror, Andrew demonstrates that not all of the men in his family act honorably. When Antonio finds Andrew at Rosie's refusing to take Narciso's warning about Tenorio wanting to kill Ultima seriously, it cuts Antonio to the core:
I did not want to see anymore. I pressed my forehead against the cold wood of the porch wall and closed my eyes […] I wanted to hate Andrew for being with the bad woman, but I could not. I only felt tired, and older. (14.834-840)
Eugenio (a.k.a. Gene) and León
The Márez blood is strong with these two. Gene and León return home, take off, return home, and then take off again. They just can't stay in a small town after having seen the world while fighting in the war. Also, they're shaken by what they've seen. And since they can't shake the war, they feel like they need to just keep moving: "We could go to Denver, Frisco, hell the sky's the limit!" (8.79). Easy there, Gene.