My sisters Cat was killed by my Neighbours Dog, what can we do?

I am sorry that your sisters cat was killed. But there is nothing you can do about it. The dog was at home and the cat should have been also. I know how cats are.They like to roam about but you can't let a cat roam into someone else's yard and then when it gets killed try to have the dog put down. I know you are upset but that is a little harsh wanting to kill the dog. It did nothing wrong. It isn't like it came to your house and killed the cat. That is their instinct after all. If you are wanting someone to be mad at I think you should be mad at your sister because she did not watch the cat when she let it out.