Hi mats.
I'm sure they are well aware that it's a mess but the chances are they now find it so overwhelming they don't know where to start so it is easier to not do it at all. If they have both suffered from ill health recently they may well just not be up to it. There are many cleaning services around that actually do a 'spring clean', they are not that cheap but is it at all possible to get one of those in along to get it back to scratch? You may find that once the worst is done they are then able to cope. Alternatively see if you can find a regular cleaner to go in once a week to do the bigger stuff; bathroom, vacuuming, washing kitchen floor, changing beds. It sounds as if they are unable to cope and telling them it's a mess may make them feel worse rather than galvanise them in to cleaning it up. I', afraid I think probably practical help is needed rather than telling them. How old are they by the way?