SInce there are 7 heirs they are based on the 7 sins, we got Pride for the first Prince, the Second is probably Wrath and everybody else kinda introduced themselves. Jilbagias's place would be Greed. Then again he is full of wrath so him being wrath while the second being greed can work.
I think Jilbagias is Wrath.
In a previous chapter At the end of ch 9 where they first introduced the siblings, I believe the older sister (Rubifia?) was the one who was shown with a chest of jewellery, so would more likely be Greed. This would also align with the story the Demon King is telling Jilbagias on the way to the meeting room.
It’s also then possible, that in each of the Demon King generations there are always 7 heirs and they represent the sins (and that perhaps both Jilbagias and the current King are both Wrath…?), hmmm 🤔 🤔 🤔…
Pride - Aiogias? (1st)
Greed - Rubifia? (2nd)
Lust - Diagias? (3rd?)
Envy - Emergias? (4th)
Gluttony - Spinezia? (5th)
Sloth - Topazia? (6th)
Wrath - Jilbagias? (7th)