I'm going to tell my step-dad to skip church to stay with his dying cat
Is there a reason why you think that he should stay home?
Yes, the cat is very old and can pass at any minute.
I think letting him know your opinion is the only thing you can do here. If he does not want to stay and wants to go to church, then there is nothing else you can do.
He's going to regret it when he loses his cat. I'm going to see to it that he doesn't miss out on important things in his life.
If he ends up going then that is that. It is good you want to tell him about the cat, but at the end of the day, you can't control him.