Cat weeing indoors all of a sudden

[Thekitten]

I could do with some advice here. I'm currently staying with my parents with my two cats (not hugely relevent), and they have a cat of their own. Their cat has always gone outside for the toilet since she was 6 months, but in the last year has started toileting indoors (before I moved in with my cats). As I brought litter trays with mine, and after scraping poo off the floor and cleaning the floor, I set one up for my parents' cat to use in the same place as she pood. And lo and behold, she used the litter tray all week. I cleaned it out, topped up, etc. No more accidents.

My parents come back from holiday and ask me to get rid of the litter tray because it smells. and she 'only sometimes toilets indoors' and 'having a litter tray indoors will encourage her and we want her to toilet outside'.

So I got rid of it. She pood indoors the other day and weed on my parents bed last night. They still don't want a litter tray despite her ruining the new mattress and duvet. (They normally have a mattress protector but is in the wash).

So... What do I do? Leave them to it or set up the tray anyway and carry on with what I was doing?
She's 13 years old, could she be ill? She started doing this when she was bullied by a neighbour's cat but that's since gone away.

I'd set up a tray. Mich easier to clean that out than scrape it off floors and beds. Mine mainly toilet outdoors but we still have a tray

Thank you pixie
My two aren't really using their that much so I'm tempted to just move one and make it my parents' cats one, but a third tray might be better with the three cats.

Arthritis
Stress
Would be the top two causes I would consider in a 13 year old. Both which things can be done to help.

Well she definitely needs a tray. She's at an age when many cats decide they no longer want to go outdoors. And trays don't smell. Wood based cat litter has no smell, and if you get rid of the poo as soon as it appears will continue to not smell.

lonecat stress is possible because it started when she was bullied by another cat, but dad put in a microchip cat flap stopping the offender getting in. He's lost interest now but she's continuing.
Didn't think about arthritis. I'll research.

@thecatneuterer I know that, you know that, my parents on the other hand. I've put a tray where I had it last time and we'll see how that works out. I don't use anything other than woodchip, I love it!