I'm thinking it could be the daughter (the one that was being watched in the bathroom).
Is she the one that came home late on the day of the murder, with an excuse that supposedly hadn't been passed on to mum?
There's bound to be a "twist in the tale". Perhaps the daughter is trying to set up one of the lads? Pinched his trainers and then after the murder, smeared them with blood and hid them under the bed, knowing full well that mum would find them?
Had previously had a bit of a fling with him and was jealous when he turned his attention to another girl?
I'm looking forward to tonight's conclusion.