That's a short story, but what a nice ending. Even tho' some things were left somewhat unexplained like "Why did the little brother tried to kill his own sister?". I hope he was manipulated by the prince and didn't simply agreed, out of anger.
(After re-reading chapter 1...) In this chapter, Mu yi told his sister 'If I were you, I would rather have drowned in the river~ "Sister" '
Tho' we can't be sure if he was sincerelly wishing her death, because near the end he said everything was just a provocation...
(After re-reading chapter 3...) In this chapter, she receives memories from the possible future of her own death and tries to find the white haired master to avoid that fate. But in this realistic vision she had, I can't find a really big motive for the little brother to help the prince, even in that timeline. I don't wanna hate him, I hope there's a good motive for that.
Maybe the key to avoid that "bad ending" was the protagonist to also be friendly with her (?step?adopted?-)sister. Maybe she has big impact on that scene even tho' she doesn't appear there.
But, oh well, we will never know unless there's a novel with some answers, which I have no interest in searching now, 'cause I've alread read the manga until this point. And it was a good manga with good withing and principally graphics.
I'm just not sure about Mu yi, 'cause there's no worst sin than killing or having the intention to kill one or even a thousand persons. There's some exceptions, but in this case, she didn't do anything that bad to deserve dying.
I don't care about the prince, but Mu Yi, I just can't forget what he did, and now he's like a completly different person. And I don't think It makes sense to kill someone you love just because you can't receive affection from her.
And also, that timeline with the "bad ending", nobody knows what happens after she gets stabbed by a sword, like "Has she survived that?", "Does Mu Yi regret something?", "Is the prince killed on the spot?", "What is the (?adopted-)sister reaction after her death?", "Will the alive sister marry the prince?", "Will she know who killed her sister?"......
Since the world was the world of a story, and the prince was the male lead and the "Novel protagonist" (aka. adopted sister) seemed to be in a some kind of secret relationship with the prince, I bet that the sister married the prince without knowing anything, or maybe it ended up in a tragic ending where she kills the the prince whom she loved with her own hands. There's so much possibilities. Because I also don't want the sister to have married the prince no matter what timeline it is.