Another reason that we suspect nephew (I keep remembering things that help the puzzle pieces come together)...it was him who "discovered" the dog food in the cheesecake. How would one even know to look there unless you knew about it before hand?
Also, DH said he checked in on the boys before he left for work and he said DS, though awake, could hardly keep his eyes open, further corroborating DS's story.
Nephew has been pretty heavily affected (more so than his sisters) by the split of his parents. Like I mentioned in the other vacation post, ex-SIL was successful in turning her son against his father where for a period of time, nephew was refusing to go to his house for visitation and blaming his dad for everything. Also, two weeks ago, his dad's girlfriend had a baby. A boy. Nephew had made it quite known that he didn't want this baby to be a boy as he wanted to remain his father's only son.
Not definitive proof by any means but it might help give some additional insight as to why we do suspect nephew was involved. Attention seeking behaviour perhaps?