While Jax did have a brother, Thomas, he died of his heart condition when the two were just children. It isn't likely that Paul would play Thomas because he's too old, unless it were some vision of what he would have been like had he lived.
It isn't out of the real of possibilities that the two could be brothers considering Jax already has a half-sister in Ireland from his father, John Teller. Whose to say that Gemma didn't also have an affair and gave the baby up for adoption so there wasn't any evidence that she was cheating on her husband with Clay Morrow?
Many speculate that Aaron Paul would likely be more of an upstanding citizen that gets sucked into SAMCRO in some way. Whether it's that he's bribed, is drawn to them because of familial ties, or some other reason, it's not implausible that it would happen.
Do you think that Aaron Paul could pass as Charlie Hunnam's brother? Do you think that the story line would work? Kurt Sutter thinks it will because the stories are both similar in that they are breaking the law but in a way that they still have a conscious about what they are doing.