I'm not going to tell my father that Auntie died
How will that help him?
It won't, he will just be sad because he loved her.
It's upsetting to have to hear about loved ones dying. But you should tell the truth, they'll have to hear it eventually and should hear it from family.
What are you talking about? This is not a big deal, it's just a cat.
I thought you were talking about your real Aunt who was a person. My bad, but he loved the cat like a person so I would hope you told him. Being honest is how to go about everything in life.