So sorry to hear about your recent loses. I think it's completely normal for a child to talk about death, especially when they have experienced the lost of loved ones. My 5 year old has asked if I would get a new child if he passed away and if he would be able to take his toys to heaven.
As for your cat, is it likely that she will pass away in your home or will she have to be euthanized? Our golden retriever had cancer and we knew that she wouldn't pass away in our home. We started to prepare our children in advance by telling them that she was sick and that she was going to go to heaven in a couple months. It depends on your child though. My boys handled it really well. When I was a child, I was horrible at handling death with pets. It would have only caused me more stress to give me advance warning months in advance.