I've read a lot on that case over the years, and a lot of stuff that wasn't widely reported at the time.
The general consensus is that it was the brother, and the parents covered it up so that they'd only lose one child, instead of two.
My personal "would like to know" is Madeline McCann. That poor baby.
Also there's that boy, around 13/14, I forget his name, who took £200 from his bank account, got a one-way train ticket to London, and was never seen or heard from again.
There was a girl near me, from Milton Keynes - Leah something - who just disappeared walking to work one day. Her brother ended up unaliving himself because of the trauma, and her remains were found in an attic about 3 years later. The house was owned by someone who lived abroad, and was being "looked after" by a local sexual predator, who died between Leah's kidnapping and her discovery, so her poor family never got any justice for her death, or her brothers.