I'm wondering if the female suspect was playing the role of a lookout and/or potential assistant if the male suspect required additional help. Surely somebody must be playing that role of a lookout, even as they had a good idea of everybody else's schedule.
The idea might be that the male suspect is a man, and hence supposedly much stronger than the so called "anorexic" girl. Maybe the male suspect handled everything himself. Maybe he called out for help.
My theory is she was probably assigned as the lookout person, who may or may not have helped with the actual fatal blows during the critical moments of the kidnapping/murder, if the male suspect had called for backup.
Am I correct in thinking that Becky's stepmum was at home with Becky and NM & SH before she left for her appointment?
If so, then NM and SH would have known that as soon as Becky was reported missing, they'd be the automatic suspects as they would be the last people to see her alive. Also the stepmother would possibly mention to whoever was driving her to the appointment, that she'd left all 3 of them at home. So I am finding it had to believe their story of planning to kidnap her.
I can't believe even they would be so stupid as to kidnap someone when witnesses would be able to place them at the house when Becky went missing. Ok they could lie and say she left, but its not much of an alibi, especially when they would know that nobody would be able to claim to have seen Becky afterwards, so the trail would always go cold after the 3 of them were placed at the house.
I think that something happened which hadn't been planned. I think it occurred on the spur of the moment - which would explain why all their clean up and disposal arrangements were made afterwards. If they had planned to kidnap Becky, they could hardly take her to that hovel of a house they had, there'd be nowhere to keep her - especially without their child seeing.
I wonder if the child has given the police any information - would that be classed as being 'tipped off'? Maybe innocently talking about mummy and daddy taking lots of bags somewhere, or seeing Aunty Becky 'asleep' or even talking about her parents being in the bathroom making lots of noise. I find it hard to believe the poor child didn't see or hear anything during the killing or disposal of Becky. It must have taken hours to do those awful things in their home, and if the child was there the entire time, she must have seen and heard things.
I don't know why they have come up with a kidnap story, but its obviously a 'nicer' story that what did happen - which is chilling.
And no way did SH not know what was happening. A person fighting for their life would make a lot of noise, over a period of time. To inflict 40 injuries, that would have taken minutes and minutes. And, if you only helped after an accidental death (as is their story), surely you'd get upset hearing about the attack and injuries Becky suffered? The fact that SH didn't react at all makes me think she was totally involved.
All just my opinion.