My aging parents have been freely feeding their 10 year old cat to the point where she has become morbidly obese. She has begun urinating and defecating all over their house, and they are planning on taking her to a shelter, where she will almost certainly be euthanized. They are aging and disabled and cannot care for her properly anymore, so I am contemplating taking her in myself. I worry that she will do the same to my apartment, but I want to at least try to help her before sending her away to her death. Aside from her chronic weight problem, she's still a healthy cat. She's vocal and can still move when she has to, but has become quite lazy. I believe the reason for her doing her business all over the house is because she doesnt have her litter box changed frequently enough, and my parents just aren't capable of giving her the attention she needs anymore. I also have her sister, whom I've taken care of for the past 10 years and she is in great shape. She doesn't need to lose weight, so how can I have both cats, but only keep one on a strict diet?