How I think Primo Secondo and Terzo would react to finding out Copia is the new Papa.
Primo was told first. Nihil knows that telling all three of his sons at once would lead to yucky emotions he doesn't want to deal with. So him and Imperator tell Primo separately and task him to tell his brothers. Primo is smart enough to notice the favouritism Imperator has for Copia. The slight resemblance they both share. So it's not a complete shock being told he is being promoted to Papa. He still does feel betrayed by his farther for choosing someone who isn't part of the bloodline and going against tradition but Primo knows the choice has been made and there's no point in arguing. It is what it is and he has to accept that.
When Secondo finds out he cries. His strong and intimidating facade crumbles before his older brother. All the bottoled up emotions come crashing down on him all at once. He's angry furious sad. He's experiencing all the frustration he's felt in his whole life. I have a personal HC that Secondo wanted his farther approval more than anything hence why he drank and partied so much during his rein, he wanted to imitate Nihil to have common ground. Now he has to watch as the time he wanted to spend with his dad being spent with a cardinal who he doesn't know anything about. Extremely jealous seeing nihil and copia walking the halls together. He doesnt yet know about Nihils distaste for copia. All he sees is another replacment. All primo can do is hug him, rub his back and try to console him. For Primo it hurts more seeing his brother in pain.
Terzo sees red. He is furious. He goes into a complete rage. A cardinal. A nobody. He is being replaced by a complete idiot who can't even say a complete a sentence with out stuttering or fumbling over his words. He didn't win a Grammy, Terzo did. He doesn't have stage presence, Terzo does. Why is he the one getting replaced by a man who is so far beneath him? It takes a while for Primo to talk his brother down from his anger. A few brocken bottles need to be cleaned up after it's all over.
After all three have been told they go drinking together. They get the most expensive stuff they have and hide out in the greenhouse for a few days. There's a lot of yelling crying and comforting. It would be a while before they attempt any positive connection with Copia.