You kind of just said what I've been saying the whole time. Yeah, the person that does it is already powerful enough to do it, but it's just about showing someone else that they can do it; making them feel the effects of it; in their minds, "putting them in their place". There's a difference between knowledge that you can commit the act and actually doing the act. I know that I can make an accurate shot from great distance with iron sights, yet I go and do it nearly every weekend because I like to display that ability; show them that I've actually got the "goods" so to speak. Yeah, that's a really, really poor example...but it's the thing that first came to mind. You can see it in everything from sports to video games (I defy you to refute the power/dominance thing when it comes to XBL/epeen issues). It's a popular theory because it rings true.
Okay, the brother wanted to have sex with the sister, she didn't want to, but he did it anyway. How is that not a power issue? We're arguing semantics. It was him saying: "I don't care what you say, this is the way I want to do things, that's how we're going to do them, deal with it." Yeah, it was a sexual crime, but the motivation behind it wasn't purely sexual. With the coaxing (again, I'm a criminology student, currently an intern with the investigative branch of a mid-size midwestern city), it kind of makes me think that he's a "game playing" criminal; again, these are people that like to show their power, but do so in a way that gives them a heightened experience due to the fact that they feel like they've solved some kind of psychological/social labyrinth. It's the same rush that we as gamers get when we figure out a new strategy.
The father being an abusive prick is just another piece of the puzzle to slide in. No wonder that the brother adopted a sociopathic mindset, and if the father displayed violence towards the sister or the mother, it only served to instill the belief that this kind of behavior is the norm. That doesn't make it excusable, but it's likely a pretty accurate interpretation of what went down psychologically speaking.
Now, are some sex crimes truly sexually motivated? Absolutely. Deviant fetishists are a prime example. I sincerely do not believe that child molesters and the ilk are 100% power driven; they have a truly warped attraction that was created via a number of possible reasons. Some rapists apply to this. There is no "one-size-fits-all" formula for crime, no matter what CSI or NCIS presents; so there is margin for error and wiggle room when an investigator seeks to apply a hypothesis as to the motivation for a crime.