First of all, Sirius isn't all that complimentary to people he doesn't like. Anyone recalling the Snape Crusade? I wouldn't be surprised if he just discounted his brother as someone intelligent just because he didn't like him. Also, Regulus would have access to all kinds of dark magic. His family is up to their eyeballs in it; it would be ridiculously easy for him to discover what Tom was really up to.