On a very surface level the boys have the same mother—not the same biological mother, but the same mother who is raising them. Mark's parents killed each other in a drunken fight. Ever since then, Mark has lived with Bryon and his mom. They are foster brothers.
Early in the story, Mark and Bryon are inseparable. They like talking about the same subjects, hanging out in the same places, and doing the same activities. I'm not saying where they hang out and what they do are positive things, but they still like them all the same. For example, one of the things that Mark and Bryon like doing together is hustling pool players at Charlie's bar.
Both boys care deeply for Mrs. Douglas (Bryon's mother). Mrs. Douglas has a few hospital stays, and one of them lasts for quite awhile. That hospital stay puts the family in financial trouble, but both boys seek out ways to earn money to help pay the bills. Granted Mark's income is from selling drugs, but he is doing it to help out Mrs. Douglas.
On a deeper level, both boys experience emotional shallowness. Mark experiences it first. His first expression of this is when he admits that he cannot relate to the black girl that was beaten up and lied about her rescue. He says that he would hate someone like that forever, which is how he feels about Bryon when Bryon tells the police about the drug dealing. Mark also struggles to understand why Bryon is spending more time with Cathy than him. Lastly, Mark cannot come to grips with why Bryon won't spend more time with the gang and engage in violence and revenge. As Mark ages, his emotional maturity never grows up with him.
Conversely, Bryon at first appears to be growing up physically and emotionally. He begins falling in love. He distances himself from the gang life and the revenge that it encourages. He gets a paying job. His sense of right and wrong becomes an incredibly strong force in his life, which is why he turns Mark in for selling drugs. However, this is when his emotional journey stagnates and even reverses. After the trial, Bryon distances himself from everybody, including Cathy. He tries to apologize to Mark, who responds by telling Bryon that he hates him. This drives Bryon further into an emotional shell of his former self. The news that Mark is being sent to prison fails to affect him, and the story ends with Bryon being unable to care about anything and wondering if he made the right decisions. At this point, the reader feels that all of the progress that Bryon has made is completely undone.