I don't know what to do about my Sassy

Tweet

The Reader's Digest version goes like this: We got her when she was a kitten and we had an older cat who never really accepted her. Within the first year after getting Sassy, the older cat ran away and never came back so for about a year, Sassy was the only cat in the house. Then Mom brought home a starving stray who was also very unaccepting of Sassy. We found out later that it was a hormonal thing (she was pregnant when we brought her home with a litter of 5) but by the time the kittens arrived, the damage had been done. Sassy was terrified of the new kitty and spent most of her time either hiding behind the TV in my room or on top of a counter, out of reach. Before we got the new kitty, Sassy was sassy but didn't have any overt behavioral problems. After she got over being terrified and finally found the floor for more than a few seconds at a time, she camped out in the bathroom, including going to the bathroom on the rugs. I know you aren't supposed to punish cats for going outside the sandbox so I didn't. I didn't catch her going very often but if I did I would carry her to the box and put her in it... not meanly or harshly, just showing her that it was okay. Then I gave her her own box, which was great for a while, until the rest of the brood found it and started using it too. We moved to a new house shortly after that and Sassy, the mama kitty and one of the kittens came too (of course). The new house has a screened in porch and for a while after we moved in, while the garage was being painted and fixed up, the box was right outside the door on the porch, level with the rest of the house. After the garage was finished, Mom moved the box out to the garage, two feet lower than the rest of the house and subsequently down two steps (Now might be an appropriate time to mention, Sassy has some problems with her hips that make going down steps difficult for her...). While the box was on the porch, she was fine. She used it regularly and didn't go outside of it (that anyone saw, anyway). As soon as it got moved to the garage, she started seeking out corners and now has one main corner where she does everything and a couple of other spots where she pees. Mom is getting really upset about it and threatening to take her to the shelter. Other than this, she's a good kitty and it literally makes me cry to think of her in the shelter (or being put to sleep). I have read a lot of suggestions, including separate boxes for each kitty plus one extra, putting the box where it is convenient for kitty, putting foil in the places she's going...and I have made these suggestions but Mom doesn't want to try any of them. She thinks Sassy is just being bad and should go away. Or should just magically behave. I want to try to fix the problem but I don't know how. I kind of resent the other cat for stressing out my kitty but I love her kitten too and she is growing on me so it's hard to be mad at her or say she should go away because Sassy was here first. Have any of you had a problem similar to this and what did you do to fix it? Did you fix it?