My cat wont use the box.

We have 3 cats. My cat Cleo is the oldest of all of them. My brother had a cat, Princess, when we got her. Well he moved out last year with his cat. My sister has two cats that she got after I got Cleo. It seems like the trouble started then. Cleo used the box when she was a kitten and everything. Now she doesant want to use it. We had a box for each cat in the past. Then I put a litter box in my room so Cleo would use it. Now I have an electric one in my room. Its one of those litter maid ones. Well she used it for a little bit, then went back to her bad ways. My sisters cat Diana would sneak in and use it. I guess Cleo saw her or something and now doesant want to use it. My brother had a similiar problem with his cat Princess while he was living here. She would mark places. She even got his laptop. After he moved out an took her with him, he has not had a problem with her. I unfortunately am unable to move out at this time. I am not exactly sure what to do with Cleo. I think moving out would be good for her and me, so she would be the only cat, but I am unable too.

Sounds like you could have a scent mark problem that was previously established by your brothers cat......