Rebecca- Maybe the thread should not have gone in the direction it did but NO ONE specifically said it was your sisters fault, especially after you explained the cat is an indoor cat but escaped.
But you mention this in your original post: "That's so mean. I just don't understand"
So our replies are explaining A. why your cat might have gotten into posion. B. Why someone is killing the neighborhood strays in your area and C. While totally 100% wrong, why someone would start posioning cats. So you understand. It SUCKS what happened to your sisters cat, but then it also can be used to open peoples eyes to the very real problem North America has today with strays.