I just wrote a long post to have it timeout on me - here's it in a nutshell:
All my opinion only, ok:
The brother could have had some abuse or bullying issues with the sister and the family maybe knew but didn't think it was that serious. I do NOT think this murder was over a video game or what to watch on TV. There seems to be some confusing family issues going on and who knows what was happening before this murder, I am not and will not judge because I live in my own glass house. I think this anger was building up, he had thought about it and did what he felt was justified in his own mind had to be done - and he confessed, I believe that.
His story did not add up - they maybe took the TV to check a possible alibi that he was watching TV at the time, only to find out the TV wasn't turned on then, for example. I believe that can be checked depending on the TV.
There could have been evidence such as blood in the washing machine, wasching machine had been used while parents were gone, clothes were burned possibly, evidence that he left the house before coming back home (to get rid of evidence) blood in her room with his prints, no tracks in garage or concrete outside (and their place looks outside to be dirty/muddy), no scent picked up outside the house. I mean there are a hundred and one clues that don't make sense that the police might have.
The police did what they could to eliminate him as a suspect, knowing full well this wasn't going to end that way. Calling in dogs, diving in ponds, the PCs etc. They knew this brother was a danger to the one person he murdered so as long as he stayed put, no one else was in danger. I wonder if the parents were updated on what they were concluding or if hit them all at once. When watching the YouTube video that someone a few pages back posted the link to, it seems the father was more upset than just the tragic death of his daughter - IMO - he had some things in his mind as he stumbled with what he told the reporter.