In addition to the vet check up, some other things to think about:
- did you change the litter brand?
- move the litter box?
- add a litter box, since you added a cat? Yes, it was a year ago, but cats can be funny like that!
- has the 13 month old been fixed? Males do give off stronger odors on their pee and poop, and if he recently used the box, the litter may not have neutralized the odor enough and she doesn't like it!

- did you change their food? Something in the new food may not agree w/ her and gives her an unpleasant feeling so she just goes where she is.