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.
I think what happened was. Celes obviously grew up bitter towards Sera for the whole 'destiny thing' and was eager to prove that she was the 'one' and not her sister. So when Alaina made her plan known of trying to seduce the royals/nobles. I imagine Sera didn't partake, while Celes decided that it was the best moment she was going to get to leave and prove that it was her that was destined for big things. And joined up with her in an attempt to get with the crown-prince. With her essentially going
"I'll fucking show you whose destiny it is." to her lovely sister.
- Fake Regis is fucking some elven baddies during the expedition like a degenerate. Making bastards with his favorite.
I'm assuming his favourite was Celes, or why mention he had a "favourite" to begin with. And so she thought she was finally proving that it was 'her' destiny all along and not her sisters. Children with the heir = being obviously an absolute win.
- 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.
Yeah, so this is the kicker. It's either she
wasn't aware of the swap, and is pissed that her plan with 'her prince' didn't work out, and depending on how she felt about him. Upset that her lover died in prison as well (which he obviously did not.) While her sister just got to swoop in and get the 'actual prince.' Which I imagine, will lead to her somehow being
even more pissed off at her sister than she usually is if she finds out the truth. For taking the father of her children (potential lover as well), and her destiny along with it for her own. Like I can't imagine her
not being mad if she finds out the man she worked hard to get into bed with, and have his children. Only to then be cast aside once you were told it wasn't a good plan anymore by Alaina. And to then just think he died, but have it turn out that he didn't die. He just became someone else, and then the Sister you've been contending with your whole life, just swoops in, marries him and has his
'legitimate' children...She's GOT TO BE PISSED, if she doesn't know and then finds out.
Or she was/is in Alaina's inner circle. And
is aware of the swap, and has therefore had some time to come to terms with it. But I think, is still going to be rather bitter about losing 'her man' and her chance at being the one with the big destiny to her sister.
So yes, I think it's not about "stealing her husband" exactly. But the destiny that she thought "that husband" was supposed to be a part of. So losing out on him to her sister would be a rather large issue.