Don Corleone's eldest son, Santino (called Sonny), is kind of a slob. (The part is played by James Caan.) He's not inhuman or a total moron or anything, but he's a less-than-ideal Don. He cheats on his wife, he takes things too personally, and he would have no problem with getting the family involved in drug dealing, if his dad weren't there to stop him.
On the other hand, he cares about his sister, Connie, enough to beat her husband, Carlo, to a bloody pulp after he injures her. In fact, he's lured into a death trap after Carlo viciously beats Connie again, as a ploy to get Sonny out of the house. Assassins blast him to death at a tollbooth. So, he's a complicated guy—or, really, an uncomplicated guy.
Don Corleone himself says that he didn't think Sonny was a very good Don (he temporarily took over while Vito was recovering from an assassination attempt). Sonny contrasts strongly with Michael, who clearly loves and cares about the family, but is able to be impersonal and business-like when he needs to be. Michael can keep his cool.
Sonny can't, and in a certain way that proves to be his undoing. His emotions lure him into a trap, and he's unable to step back and consider what might really be going on. It's understandable, given what's happened to his sister. But it doesn't demonstrate the cool, calculating, businessman's mind that Michael has.