Anyone who condones the mistreatment of any animal, deserving or not, does not deserve to have any animals themselves. If you have outdoor animals that you want to keep safe, build a fence. How can anyone complain about kitty poop? Geez, it's just so itty bitty people. You wouldn't go around poisoning little birds for pooping in your yards, would you?
Anyways. I am very sorry about your sisters cat. I lost one a few years back and I still miss him. He was the best cat ever. We have two more now and they are great
I hope that kitty pulls through, but if not I hope your sis will be sure to give her time and affection to another someday