Sister's Cat: Peeing on the Bed (Stress or Health Problem?)

So my sister is awaiting her cat's test results for a peeing problem... but if anyone has experience with the following problem, please share!

Lulu is about 8 years old, healthy and very friendly. 1.5 years ago, we moved to a new apartment – and moved in with another cat, also about that age.  We read a book about bringing adult cats together and followed all the instructions, keeping them apart for months. For a year, things were fine. The cats don’t like each other. Lulu is often hissing at the other cat, and the other cat playfully attacks her. We always thought he was stressing her out.  Six months ago, she suddenly started peeing on our bed. Almost daily. We started to put a plastic drop sheet over our bed whenever we weren’t in it and for months this worked. Then last month, she peed on the bed while I was on it. The following day she peed on the plastic cover, and it soaked all the way through. We got a better plastic cover. A few weeks later, I wasn’t careful in putting it on and left a few inches on the side. She seems to have pushed it aside more and peed on it. Then last night she snuck into the room and peed on the bed while I was sleeping in it. Also last week we discovered her peeing into our potted tree (since covered up). 

Thank you!

How many litter boxes do you have? A rule of thumb is to have one more than the number of cats. So if you have 2 cats, 3 litter boxes. Make sure the cats have privacy from each other so the aggressive one can't ambush the other. And keep the boxes clean. Where does the passive one poop?

Am I understanding that everything was 'fine' for a year and then the bed peeing started? If so and nothing changed as far as litter box placement, etc. it may be a medical issue. It sounds like she did the right thing by taking her to the Vet. One of our cats did that a couple of times when she was in pain.

Good luck

It sounds like it may be a habit formed on top of something else. Every sheet that she has peed on and the mattress need to be cleaned with Nature's Miracle or some other enzymatic cleaner. To her, they smell like a litter box.

Have you had her checked at the vets?  A change in pee habits can often indicate a health problem.