Sinner is a standalone companion to Priest about Father Bell's brother Sean. You do not have to read Priest or Midnight Mass to read Sinner

I’m not a good man, and I’ve never pretended to be. I don’t believe in goodness or God or any happy ending that isn’t paid for in advance.
What do I believe in? Money. Sex. Macallan 18.
They have words for men like me—playboy. Womanizer. Skirt chaser.
My brother used to be a priest, and he only has one word for me.

Sinner.