I think she has been remanded in custody for her own safety, and although maybe the step uncle did not abduct her and was waiting for the mother to join him why didn't he just come forward and say he picked up Shannon not realising the trouble it had caused and act all innocent? The Police would have probably "slapped him on the wrist" for not telling anyone and no harm would have been done. So I do think he was well involved, did this "family" really believe the Police would give up looking or not take them seriously and then not ask questions once she was found.
It is quite dispicable the lengths some people go to in the name of greed. Karen Matthews should be given life and be sterilized she is not fit to call herself a mother.