I live with my DC, my parents and my nana, as well as an assortment of pets.
We're dog sitting atm so my cats are 'living with' my nana in her living area. Possibly because of the dog the cats seem to be weeing in the living room. (They have a clean safe litter tray but they are possibly stressed). Nana can't smell this and they are my cats so she reluctantly agreed to me using some special cat smell products (actually just the anti bacterial carpet deodoriser and a fancier Hoover) to clean, so while she was at church I went in and pulled all the sofas out etc, cleaned the bathroom with bleach and so on.
It is clear she hasn't been cleaning properly for a good while - including things like food prep surfaces etc. It looks ok at a casual glance but it's things like when I wiped a bit of the sink the bits that just looked like wear and tear wiped off easily.
She's really proud but is physically and mentally getting frail, and keeps getting stomach upsets and so on. She also gives the DC and visitors snacks from her kitchen and contributes eg sandwiches to occasion meals for the house. It's also a vermin risk, especially as we have a lot of pets.
We think she just can't see/smell/manage physically to clean properly, but won't accept help because at 86 she doesn't feel old enough to impose 
Would I be unreasonable to make a regular thing of going in when she is out or in bed to give it all a good clean? She knows (and encourages) that I sometimes sit in her living room after she goes to bed to watch her tv or whatever.
It feels intrusive, but also its gone past getting behind with housework (and I have really very low standards myself) and I don't feel she is able to keep it at a safe standard.
We already keep an eye on her medication and whether she is eating and drinking, but she doesn't mind us 'making too much for tea so bringing her some' or 'making ourselves a brew anyway' and we all make sure she gets plenty of company and trips out.