The three Curtis brothers are Darry, Ponyboy, and Sodapop. The relationship between Soda and Pony remains fairly static throughout the novel. That doesn't mean the relationship is boring or anything like that. It simply means that not much change occurs. The interesting relationship is between Darry and Ponyboy.
At the beginning of the novel, Ponyboy feels that Darry doesn't love him as much as he loves Soda. Ponyboy feels a bit left out. It's not Darry's fault really. He's been left to care for his two younger brothers like a parent, except that he's a teenage boy. He's not a parent. The reason that Pony feels this way is because he believes that Darry is exceptionally hard on him. Pony feels that Darry is always criticizing him. Pony feels that it's always negative. He never once considers that Darry is doing his best to make sure that Pony becomes all that he can be. Darry sees ridiculous potential in Pony and wants to hold Pony up to it. Pony just sees criticism. The situation isn't helped when Darry hits Ponyboy. That's when Ponyboy decides to run out.
Several chapters later, Johnny is severely injured in the fire rescue. He is taken to the hospital. Darry comes to the hospital to see how Johnny is doing and completely breaks down into tears. At that moment, Pony realizes exactly how much Darry does indeed care for him and the rest of the Greasers.
"In that second what Soda and Dally and Two-Bit had been trying to tell me came through. Darry did care about me, maybe as much as he cared about Soda, and because he cared he was trying too hard to make something of me. When he yelled "Pony, where have you been all this time?" he meant "Pony, you've scared me to death. Please be careful, because I couldn't stand it if anything happened to you."
Darry looked down and turned away silently. Suddenly I broke out of my daze.
"Darry!" I screamed, and the next thing I knew I had him around the waist and was squeezing the daylights out of him.
"Darry," I said, "I'm sorry..."
He was stroking my hair and I could hear the sobs racking him as he fought to keep back the tears. "Oh, Pony, I thought we'd lost you... like we did Mom and Dad..."
That moment is a turning point in their relationship. So much so that when Pony is in a delirious state late in the novel, he asks for Darry. His mind never would have done that 200 pages earlier.