I can't see how they would of slept through Hartley, I have 4 boys under 11 and we wake in the night my husband and I know not to flush the toilet as that alone wakes them? Why go for the twins first though?, they were the least threat in the house? No one went in that room after they were murdered as there was no blood smears/spatters on the carpet so not one of the adults checked them during this? Why not? They surely would of been first port of call for any caring mother/grandparent?
All kids are different I guess, but my little one will wake at the flush of a loo also.
We'll never know, but I think that there are scenarios that would fit with the twins being the first victims, there may then never have been an opportunity for one of the adults to check on them.