But

Well, she's actually a princess from a fallen nation. In the first chapter she begged to be a maid instead of being a royal hostage

In another manhwa titled “Beatrice” the MC was a slave turned princess then became a slave again, but because of her old status, she was technically able to marry the ML who was of royalty and prove her standing. So I assume MC in this story just needs to flex her old status a bit, though there might be some difficulties but it’s not impossible in these types of comics.

Sienna is the last royal of her kingdom, if it's known she was alive, her people might try to bring her back and reclaim the throne. So, the ML could just make it like he took her to prevent that. Also Sienna might be naturally accepted in the new empire since she already made friends with the royalties there, the 2 princess and that 1 consort with his son.

She's also endearing herself to some of the royal family, though it's not really her intention. So I'm sure that'll help end game. She might she. Some push back, but it shouldn't be too bad. I'm sure that by that point, enough of the royal family members will be okay with it due to her assistance for their issues.

You’re right! With all those side quests gaining the favor of all those royal members, her chances of bagging the ML is pretty much a done deal, the girl has got actual backing (currently have 1 concubine, 2 princesses, maybe the empress dowager, and naturally the ML)(￣∀￣).