The advice to say and do nothing is understandable, but I am not sure that it is the most suitable in this instance.
If you decide you must say something, which I do understand that you probably do, choose a time when both your son and your daughter-in-law are present.
It might be wisest to start by offering to help. Decide beforehand what help you will offer and be very specific.
It will do no good just to say, "Do you want help cleaning?" Presumably the adults are quite comfortable in this mess.
You need to state politely the reasons for your concern, and for what can be taken as interference.
I am a cat lover myself, but would never have five cats in a small flat, nor allow any cat of mine to walk or sleep on a ktichen work-surface of any kind or table.
Unfortunately, these cats have been allowed to do so, so it is well-nigh impossible to stop them now.
If they are using the bathroom floor instead of the litter boxes, this is due to one of two things.
Either the boxes are not changed often enough or one of the cats objects to using the same box as any other cat.
A lot of people believe that it is sufficient to removed soiled or wet litter once a day and occasionally fill the box up with a new layer of litter.
In my experience doing this encourages cats to find an alternative. The box should be completely emptied, washed in warm water and detergent and left to air-dry completely before being refilled at very regular intervals if you want they cats to be satisfied with its condition.
This means that you need a couple of extra boxes and a strict cleaning rota.
The rubbish bag isn't an issue, as long as it is changed daily.
Mould on the other hand is a serious health issue and should your relatives want to move at some point they will find that no-one is willing to buy a house with obvious signs of mould.
Cat hairs on upholstery and carpets are unsightly and may increase the risk of astma or allergies. Very few vacuum cleaners deal satisfactorily with removing them, even if you have what is supposed to be a mouthpiece designed to remove cat and dog hairs.
Once or twice a year, I use an old plastic pot scourer wrung out in cold water to remove cat hairs from our sitting-room carpet. This necessitates working on my hands and knees, but I find that preferable to an overgrown carpet.
Small rugs and mats can be brushed with a very stiff brush.
If you are fit and well you could offer to do some of all this, or at least buy a second set of cat litter trays.
However, take care - this is the kind of discussion that can cause very bad feeling between mother-in-law and daughter-in-law.
I would start by saying, "You'll probably want to call me an interfering old besom, but I am genuinely concerned by what I see as health hazards. I am willing to help with cleaning if neither of you have time to do it regularly." and go on from there.