There it is. Called it, they wouldn't have been enemies for long. If the demon lord looked completely monstrous it would've been another matter, but they were too good looking to be some throwaway obstacle. We also got the "they're not actually evil" trope here with the demon lord saying they were just reclaiming land stolen from them.
I have mixed feelings about her becoming an underling though, as this sacrifices her agency in exchange for the military power she desired to destroy the kingdom that turned her sister into a human battery trapped in a rock. Unless her new boss will allow her some leeway for independent action.