Responses
No, it was her father/family wish for her to fail. She wants to be Empress to prevent her family's tragedy from their last life. I don't see the point in being Empress if the whole thing from last life was set up cause the older sister loved the king and the other redhead just set her up to be killed. If it was political then yeah I understand why she would want to marry him to prevent it. We don't know the real reason either cause no one thought to tell MC for whatever reason.
I don't know about you guys but, maybe just maybe, fail at being the candidate so you won't have to worry about your sister being married to that ahole? She won't even have the chance unless for some unbelievable situation where the guy suddenly sees her and decides to marry her. I mean why even sacrifice yourself unless you wanna kill him and his mistress? I would've just failed at that shit. Why would I wanna marry a dude who has a mistress already and will probably treat you like shit?