Getting 14 year old cat to stop going to bathroom in the dining room

My cat Jellybean is a 14 year old Calico. For the past 6 years she's been frequently pooping and urinating in our dining room, which is an unused room in our house. She does use the litter box sometimes, and occasionally she poops in other rooms.

She has always been finicky in the litter box, she would often go elsewhere if my mom changed to a different litter or the box was dirty, and she has never covered her feces. However, the problem has been more consistent over the past six years, which began when we got our other cat Mittens. Jellybean shared a litter box fine with our previous cat Ashley, who we got when Jellybean was four. Jellybean and Mittens never got along, Jellybean has always growled and even hissed when Mittens goes near her, and Mittens is a bit of a bully and often chases Jellybean and also chases her away from her food and steal it which we try to prevent. Mittens is also 16 pounds while Jellybean is 5.

Mittens is a complete sweetheart to us so getting rid of her wasn't an option. We tried cleaning litter more frequently, we added 2 more litter boxes including one in the room she goes in, and we try to make loud noises if she's caught. We also took Jellybean to the vet and the vet said she's healthy. Six years is a long time to be figuring out this issue now, but now my parents might put her to sleep. My mom for some reason never had issues cleaning the room which is why it took so long to get to the point where I'm actually trying to seriously figure this out.

I don't know if anyone has any advice on what to

It sounds like the cause of the problem is due to stress and I suspect Mittens has probably chased Jellybean away from the litter box before.

How did you go about introducing Mittens to Jellybean when you first got her?

Have you tried giving Jelly bean her own space with her own food/water, litter box, bed, toys, etc..to see if that gets her to using the box properly?

I also suggest getting some Feliway diffusers to have throughout the house (1-2 diffusers per room depending on room size.)

We sort of just brought Mittens in, and she was a kitten. At first it was actually Ashley that hated Mittens, and Ashley actually died 3 weeks later from cancer. Jellybean has always been the aggressive cat and her initial relationship was actually more neutral than the sweeter cat Ashley. However, I think it's because Mittens was a high energy kitten and Jellybean was an older cat, but Jellybean started to hate Mittens right away.

I've though about separating them but it's hard because either Jellybean won't stay where she's "separated", or Mittens would find a way to join in. My bedroom is where Jellybean spends most her time and Mittens doesn't come in too often, but I don't want a litter box or food in there. Jellybean actually hasn't pooped in my room except for a few occasions

As much as you may not like to, I think setting Jellybean up in your room would be my solution for the time being. A litter box in the bedroom is a small price to pay when your cat is having accidents.

I think the reason Jellybean isn't going pooping/peeing in your room is because that is her territory and it seems that Mittens knows this since she doesn't venture in your room much.

As a note, I have 2 litter boxes and food/water in my bedroom. I have hardwood floors, so use a rug to keep litter from tracking all over the place.