
The Heir thing can be due to the fact in the the past in the medieval era if your sibling was the heir it means you have more power and support and your husband gets that power as well so as you can see that is why since the prince didn't fall in love first he had chosen the other girl BUT now that he has taken a liking he will chose her probably.
[SPOILER]
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I heard in the novel the ending was open as in a harem ending.
Someone also said she was close to the heroine in a romantic way I guess.
Someone stated the ML was always saved and never did thing by herself
THEORY OR NOT
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HOWEVER,
The Genre label did not have "Harem" in it. and She only blushed with the demon prince (I think he is the demon prince)
Honestly, she seriously looks to only have feeling for the demon prince so the story might take a tern in the manga so we have an official ML.
Aw sh*t, I was getting a little sus with this cliche sprained ankle = classic princess carry. They really toss the phrase "strong female lead" for anyone who's supposed/expected to be strong when in reality... *sigh*