Although the cats seem to get on fine, their relationship may still be the cause of the problem. For example, one cat can intimidate the other simply by sitting in between her and the litter tray - he has effectively blocked her and she can't use the tray. This is just an example of the subtleties of cat relationships! Cats can also decide they don't want company after years of being fine.
I recommend you buy any book by Vicky Halls a cat behaviourist - have a look on amazon.
Zingy: cat flaps are quite safe as long as the door is locked and you don't leave the key in the lock, which a burglar with long arms could potentially turn. But it's never a good idea to leave keys in locks anyway so really so shouldn't be put off getting a cat flap.
As others have said, you probably just need a catflap or more litter trays. Biological washing powder is good for getting the smell of cat wee out of the carpet so she doesn't keep going in the same spot. You could also try another litter tray with earth in it since she's used to going outside.
Thanks. I have been out and got another litter tray. It has a smaller door so hopefully the male cat will be too big to get in. My husband worries about the safety of cat flaps as a relative of his had one and the burglars reached inside and opened the door. He loves all the animals as do I and our two children.
I'm afraid one litter tray isn't enough. The wisdom on these things is one per cat plus one - but if that isn't practical you do need at least two trays for two cats. And if the boy cat uses them, well you'll have to clean them very assiduously and as quickly as possible after they've been used.
I'd try careful deep cleaning of anywhere she's gone by mistake, get two trays, clean them a lot and see how that goes.
Does your husband have any specific reason for not wanting a cat flap fitted? What is his relationship with the cats like - because a flap would seem (from what you say) to be a good quick solution if she only pees inside when she's been kept in.
I am desperate for some advice regarding my cat. I have a dog and two cats they all get on fine together. The cats are male and female. The male is the larger of the two and the female is smaller. I have had the female from when she was about 6 months old she is now 12. She has never been litter trained. She uses my house as a toilet. Recently she has weed on my daughter's DS and our house phone. I am seriously considering having her put down. My house stinks of cat wee and poo constantly.
She is not a fussy cat and prefers to be out all the time. If she is kept in for more than a few hours she will not use the litter tray and just goes to the toilet where she chooses. I had two litter trays at one time but the boy cat just used both and then she wouldn't go near it.