In December, my wife and I adopted a 3 year old female tuxedo (owner surrender) and a 3 month old yellow male (from feral litter). We kept them in our room for two months (the room is over 1200 square feet). At first, the kitten was the problem. They fought nightly and he was usually the instigator. In February, we moved him out of the room into the main house because she had started using our bed for a litter box, despite having 3 boxes. We thought it was a territory thing. It was not.
We've taken her to our vet and ruled out urinary tract problems. She does have a skin condition that is currently being treated by our vet. Other than that, she's medically fine. She won't use the litter box for days, and then she'll jump on our bed, deficate and urinate. She also howls constantly if we're in the room and she won't sleep anywhere other than on top of myself or my wife.
We've tried attractant in soft litter. We've moved the kitten to the rest of the house, which worked for about 3 days. We've spent close to $1000 in cleaning up after her. I'm not one to take an animal back to the shelter, but I don't know where to go from here. Calming treats and collars also offer zero help.
Any advice from a professional?