Brenda and Billy are unstable on their own. Put them together, and they're basically the nuclear bomb of destructive relationships. The show tries to pin some of their weird incestuous behavior on the fact that their parents were sexually liberated and maintained an open marriage that both children were aware of (and sometimes witnessed). That doesn't seem quite fair, though, because the parents aren't the ones harboring naughty feelings for their siblings. Billy is frequently depressed, and has attempted suicide in the past. He's obsessed with his sister and becomes outrageously jealous when he sees her with other men, especially Nate Fisher, the lead character on the show. He basically can't handle not having Brenda taking care of him. Back in the 80s, he burned down part of their house so that she would leave her fiancee and come back home. And later, he cut off his half of a tattoo that they both had, and then tried to do the same to her. So...there's that.