Any cat psychiatrists here?

Did you change the kind of litter you use? If so..she probably doesn't like the smell. Are you sure the other cat didn't use her box? The other cat could have marked it as it's territory. Have you fed them anything different? Maybe she didn't recognize the smell as hers . Did you change the box or change anything? If she has a kidney etc. problem ..she might recognize that her pee etc. smells different and is trying to tell you. Could it be that the other cat peed in the corner and now she thinks that's the new cat box? I don't know that much about cats. I have 4...but they're outside most of the time and I don't have a litter box anymore. Get some of that stuff that keeps them from doing it on the floor and pour it in that corner..I know they have it for dogs. Or spray something there that you think she won't like. Try to catch her starting to do it and rush her to the litter box. That's all I can think of!