A.S.
I read this and think you you are writing about me!! I have the exact same problem. Well i did. Our oldest cat, around 10 not really sure, did not use the litter pan once we broght James home and she figured out that he was staying. We tried everything. There is a litter called cat attrack we tried, did not work, we tried pills from the vet for anxitey, did not work, we tried several litter pans in the areas she was goign in, did not work, well you get the pinot. We eventually had to give her up. But not to far. she went to my parents house and now she is perectly fine. She just really did not like the baby. She wanted her attention back and was not going to stop until she got it. If you have anyone that is willing to take her you might want to go that way. It really sucks i know. Take her to the vet to see if the anxiety pills work or if there is something else they can do.
Hope it works out and you can keep her
A.