Red-Eared Slider's Appetite?

cage? Do you mean water tanks? If so yes you can put them in the same tank, however they may pick on each other. Especially if you have a female. They are more aggressive. I know cuz I have 2 in the same tank a male and a female. As for your sis's turtle, it may be sick. I had a small one that also wouldn't eat and died just a month after a friend gave it to me. She should take it to a vet.