It can be such a nightmare trying to deal with other people's animals. Being so strict with my own pets it makes it a particular pet peeve of mine. At our old place the roomie's cat used to terrorize my boyfriend's daughter's leopard gecko and my gold fish. The snakes were away in opaque tubs, so they missed most of the little brute, but he was a clumsy cat to boot. Any pet cats I'd had myself had been like ghosts, they'd go through a room and you couldn't tell (other than some hair in their favorite sleeping spots) but this little guy would push through/past things, knocking them over. Broke quite a few of my roomie's belongings, and I ended up securing/moving any of mine away from the "danger zones"

Anyway, even if your sister throws a fit she needs to take responsibility for her own animals. This includes dealing with them when they are creating problems in the residence. I see it as your house, your rules, but I can understand with family it can get sticky (especially siblings, I'm oldest of 4). I'd lay it out clearly that the cat needs to stay in her room where she can care for it and it doesn't get into things, or it can leave. It's not up to you to work around her pet in your own dang apartment