Basically, Celestine doesn't hate Sera for stealing her "husband". It was already said by Sera that their issues started with their destiny. The prophecy told by the Dryad also didn't mention anything about marrying the King. So it would be odd anyway for that to be a reason. Especially when the Elves and Humans were still going at it during that period. Not to mention, Sera told Celestine when they were still children. Not after. Which is why Sera tells Elis her sister hated her during their childhood for stealing her destiny.
The order of events are or should be:
- Sera and Celest visit Dryad.
- Sera tells Celest about what the Dryad said. This happened while they were still "children", according to Sera. So it's not tied to the King later.
- Celest decides to leave (We know this happened because Noira and Arion were born outside of the village). She goes to the Human Kingdom in defiance? Alaina gets some elves for their plan to seduce humans.
- Fake Regis is fucking some elven baddies during the expedition like a degenerate. Making bastards with his favorite.
- Fake Regis is sent to prison on the way back after being blamed for his father death. Regis married Sybilla.
- After some time, Regis is killed by Sybilla. Fake Regis and Alaina make a deal to switch his identity to that of his younger brother.
- Alaina decides that bastards won't do with the fact that the real Regis fathered Cass. So instead, they decide to go the legitimate route. Removing all of the previously born bastards. Since it wouldn't make sense anyway. They would be too old.
- Celestine, if she's one of his mistresses, likely doesn't know that the Real Regis died and her former lover, if it was him, is actually the King. Since Alaina made sure to keep it a secret from just about everyone during this period.
- Years later, after "Regis" is older, he married Sera and then Elis and Elin were born.
So basically, unless Celestine knew about Alaina plan with the identity switch. Chances are, she thinks her former lover died in prison and that Regis on the throne is the real one. Meaning, she doesn't even know that Sera is fucking her former lover. And if all of these assumptions above are true, she doesn't hate Sera for stealing her husband. Since they weren't married and she thinks he died in prison.
There might be some errors in the information above. Since I don't have the game installed at the moment. Mostly working off of my memory.