This is second hand info but, in the original timeline seraphie used nina to try to escape, failed, and sacrificed nina to save herself, resulting in nina's execution, later on she finds out that nina was her sister which causes her to have breakdowns and self loathing, and the king is having breakdowns over giving the order "a king must make, even if he doesn't want to", making the original a tragedy as nina is the only person capable of breaking the curse, as she's the "lover" in that story that the king sacrificed, so killing her basically condemned him and i'm guessing seraphie as well, meaning they got together as a way of comforting themselves over their own sorrow over what they did, in this timeline she's still the saintess if i remember right, not much romance between them, but not sure on the epilogue.