Here the complete spoiler, do mind I read this novel a long time ago so I only remember the crucial point.
In this timeline she doing everything to keep the little sister safe, if I'm not wrong she even make a plan so that the little sister healthy, then at some point the little sister suddenly pregnant by that bastard and take her as concubine, FL despaired and hang herself (if I'm remembering the FL not even pregnant)
The next loop, she tried to ran away with the knight but someone betrayed them and tell her family their location, the knight died and she brought back to marry that bastard because there are no replacement, she had to marry that man because only she capable of taking care of some noble duties her little sister know nothing so she can't be replacement, at some point the fiancee of knight came to FL and angry at her because she make her fiance died because of her runaway action, she said they already promised themselves and FL ruin all of it . I forgot why she died in this timeline.
Then next timeline she teach all the knowledge to her little sister so that she can replace her, when the time is right she ran away alone (not bringing the knight cuz the she just know he had fiancee, she don't want to ruin her knight life), as she literally just a girl who don't know the world (all of her life is just studying of how to be capable wife for that bastard) she get caught by some slave trader and sold as prostitute. Some years later forgot if it 5 or 10, she sick as hell because no one care about her and in her dying breath she was found by her family, she was brought back to her family seeing that bastard man marry with her little sister and already had a kid and had a happy family, after some words they leave the FL and man alone for some last word because she literally dying, he said "even though I have a happy life now because of your runaway action, i don't feel grateful for you, because what you do is betrayal to me" well something similar to this, in the end she died with that knife stabbing word in the end, really when I read this line I wish I could jump out to that world and beat that shit of that man.
Well after some more despair, FL try to understand her family now, why they love this little sister so much etc. And the secret slowly revealed, FL had a new save point of her loop and in the recent chapter there already some light ahead for FL, but we don't know yet if she will really had happy ending cuz the author still writing this story, and we waiting for the updates of this novel.