Female cat with idiopathic cystitis peeing everywhere. Any advice?

[Conflugenglugen]

That's it, really. Our very highly strung female cat (who has a brother who also lives with us), has been cleared of any UTIs but has "idiopathic cystitis". We give her fresh water, we give her a litter box inside with clean litter, but she is still peeing around the house, including in the hallway, on our carpet upstairs, in the fireplace, and under the kitchen counters. Her behaviour is now becoming a liability because of the smell and the health implications.

I am running short on options, and the only thing I can think of doing - apart from selling her or giving her away to a more accommodating home (though I'm not sure there will be many) - is to stop her from coming in the main house anymore, and confining her to the laundry, which has a catflap. This means that her brother will have to follow suit.

Can anyone think of anything else before I have to resort to such drastic measures?

TIA.

Have you tried a feliway diffuser? they're mean't to help with 'stressed' cats. Haven't tried but have heard recommended.

We also have a stressed female cat who often pees in wrong place, not so bad though. Ours has got better since becoming an only cat after other cat pts (kidney failure/old age ). Also ours is now confined to non-carpeted areas (too much soiling/vomit to deal with!) tbh she seems happier in less space.

Do you have two litter trays? She might not like his smell, even if siblings. Or have you tried different litters?

I find Vicky Halls books, Cat Detective & Cat Confidential very good on cat issues.
HTH

ImeldaM, thanks so much! I will investigate further. There are so many variables and possibilities that my mind has been boggling. I think I'll try the cat litter first, and then investigate the book. Unfortunately, there's no chance of selective access to the carpeted areas.

Much appreciated!