Is something wrong with my cat?

I have 2 cats. 1 is all black and 3-4 years old. My mom gave her to me and my grandparents because my little brother was always pulling her tail. When we got her she loved to play. The other one is gray/white striped 2-3 years old. So we went to the animal shelter a few years ago and got her. She was a little 5 month old kitten. Super playful. Well ever since she came into the house the oldest one, Anubis, has been acting strange. When she goes to the bathroom, she will never touch the litter. Always standing on the side. And she hardly eats. The younger one, Mittens, is a pig! So when Mittens is eating we try to put a bowl in another part of the house and get Anubis to eat. And they do not like each other very much. They use to fight, but now they ignore each other. She has become very distant. She is not so skinny to where you can see her bones though. She eats a tiny bit at night. We put a scoop down for her, but she only takes a few bites. My 'papa' grew up with lots of cat's and he said that 'she does not have very much longer to live.' I asked why and he said 'you can just tell.' Is something wrong with her?