Aurora was likely adopted once they realized she's possibly the saintess.
Then someone, likely the dad, wanted to use her to recreate the country's founding legend of water magic using king + saintess combined power to create the holy-water barrier repelling monsters.
But Aurora being an orphan would face oppositions from other nobles, whether out of jealousy or other reasons.
So the plan was to make her Lapis, a duke's daughter so her marrying the prince would have far less room for criticism.
And to ensure she stick to the role, they probably threaten her that if she break character the orphanage might meet an unfortunate monster outbreak.
It'd explain why she's so insistent on not showing kindness to Natori, since that'd make Natori an extra 'hostage' those people can use.
Not to mention given the OG story, her plan is basically assisted suicide since she's blessed with rather durable body (she was found unharmed amidst monster attack, and despite eating very little she remains healthy)
