
The little sister is to stay at home and bring someone else into the family as the heir of the house....but the father is so jealous of her that would like for her to stay in the house..the excuse is her poor health
That is due of regular intake of poison by the mother...that too is a device to protect her, since she is another country princess and lone heir to the throne..

an illegitimate child of a woman can't be an heir to the throne as a matter of fact exposing such a thing would only bring humiliation to the queen as slut or a witch ! people of the old time were very crazy about the virginity of a woman and how she should only have sex with her official husband as a matter of fact this fact hold true to this day but now even the affair of the husband can bring dishonor and that's why you never heard about an affair from the heir to the throne
also the mc mom should love her regardless of any previous plan
this just a bad attempt to cash in the vailness troupe popularity while playing the extreme case of the victim card

such a position would make unable to be an official princess even if the whole royal family got killed and as long as the ruler of the country isn't her mom the current ruler would seek nothing but her mom death along with her husband and daughter since she soiled their blood
this plotline is only done to prove that the little sis is superior to the mc even in term of bloodline since she can't gain anything from this fact
and the plot could work even if the little sister mom was just an ordinary mistress

The kingdom of the sister's mother is involved in a rebellion...something like royalist vs some other faction....the point is that they all love the sister so much that her involvement with politics would mean danger: some want her dead and some other wants to use her as a puppet.. that is why she is caged at home and poisoned to make her think she is of poor health...
The sister is supposedly innocent...but that doesn't make me hate her the less....and Ilya family is shit....Soleil too is shit...I only hope for the girl to get happiness with crow!
This story's mc is a gloomy character that just want to be loved...and every retry she do her best...reliving your life don't make you more intelligent, and there are many factors that are outside of her expectation...
Also this is not a villainess manga, and she never try to get revenge on the fiance or the sister...she just want to escape from this loop!
japanese author really became lazy they are using the troupe of the villainess as a given thing while including only the nice things about a villainess persona but the thing that always bring them to their fall is their physical abuse and their many attempt to murder various characters
also how the hell can you convince me that the little sister didn't marry anyone after graduation which is a noble duty to secure their bloodline continue
and at least bring a good reason why does everyone hate the MC or just spell it out that she is ugly beyond belief so we can view the haters as the shallow people that they are