Why does my sister's cat hates my cat?

Cats can sense when there are changes that happen. I think what happened in this case is the neutering caused the cat to lose some of the "pheremones' that many cats possess. Because of this, it was a strange "stranger' to the other cat. I think that it will change within time, but it will take time for them to get re-introduced to each other. It is a good idea to give them the time that they need to re-establish their relationships, I know that it will eventually happen.