There is always a reason when cats exhibit this sort of behaviour but of course it is trying to narrow it down.
As already mentioned I would firstly take the cats for a check up. If all is fine physically then you need to look at other triggers. How old are your partners' boys - are they good with the cats or do they hassle them or is the dog getting too close to the cats? Have you a tall cat climber where perhaps the cats can get up out of the way of the kids and dog?
I would definitely reinstate a litter tray - perhaps a covered one, after all you seem to have a busy household now. I would invest in a couple of Feliway plug in diffusers too and perhaps put some Bachs Rescue Remedy in their water.
