I don't think they have proved the boy was murdered. The private detective thinks so. The imposter claimed it because he felt some family members knew he was an imposter, but went along with it to help hide the murder. But then he's an unreliable narrator. He claimed the sister showed him family photos when they first met, in a way that he said helped him pass the photo test set by Spanish authorities. As though she was coaching him in who everyone was.
It seems odd to me, as three years after the disappearance nobody had been accusing them of murder ( I don't think), so if they had done it, wouldn't it have felt like they'd got away with it? Wouldn't it have been simpler to say - no that's not our son/brother, he has the wrong colour eyes! Why is he talking with a French accent? (that's if they were all in on it).
Unless it was the older brother alone - I don't think he ever welcomed the imposter and on first meeting him said something like 'good luck'.
After his younger brother's disappearance he phoned the police claiming the younger brother had tried to break into the house. It was suggested this was a common way for someone to cover their tracks i.e. if older brother was responsible in some way for younger brother's disappearance.
I felt maybe the sister just really wanted it to be him, so saw what she wanted to see perhaps, strange as that may seem.
As I understand it, the guy was making calls from prison either pretending to be missing people or, in the case of a very young child, saying he had information on what had happened. Did anyone else get this? It made me rethink his entire narration, as he seemed seriously messed up, whereas at the start I felt some sympathy. And he was wanted for identity theft so many times.