Long Spoilers

Spoilers.
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So the bad guy is looking for a suitable vessel aka FL in this case, to trancfer himself into.
The reason he wants FL is because of her bloodline. The first head of her family made a pact with the "Winters arch enemy" aka the bad guy in returne for his bloodline being servants of said being. The first head of FL's family had red hair, as generations of marrages the blood thined and red hair = suitable container. The red Magic in FL is the magic her famerly was granted by the deal, the blue magic is the Winters magic which is the only magic that can hurt and seal the beings magic aka the black magic energy.
Fl and Ml are becomming happy and love eacother. Meanwhile in FL's house the sister is throwing thantrums as none of her "partys and so forth" she has hosted have been a sucsess, hench the nobels are starting to critisize her. The sister hates this and uses the exuse "they just hate how down to eart i am, and dont like i help and support the commoners!". In reality even the givavays for commoners have failed as FL was originaly in charge of all the work. The sister wants to be seen as down to earth and not as a nobel, yet loves to flaunt the "priceless gifts" the prince gives her.
Badguy ses the sister and makes a plan, as she is selfish selfcentered and greedy. He aproaches her as a "academy friend" who compliments her and every time he is near noone questiones her "caracter" like they have after Fl left.
Bad guy is inwited into the home and quickly uses his power to semi controle those of FL's bloodline aka father and brother, the mother is only tranced.
Bad buy inhances Spoiled sisters "manipulation magic" so eveyone around her does what she wants, and hence she tells the prince "I WANA MARRY!" so ofcorse the wedding procedes, and FL and Ml have to come.
When FL and Ml arive the father, mother and brother are loving to her and missed her, but the second the sister arives they all become tranched and kick FL to the cirve. Ml finds this strange as it was like they were being controled. (This shows us that the father and mothers neglect and hate for FL was actualy the sisters's doing. The sister did it subcouncious yes, but it was still her wish to have the family hate FL....such a spoiled brat).

The wedding proceds and this is where Badguys real plan is revealed. FL and Ml destroyed badguys curse on the land of winter, so he wants to turne the knigdome into his new land of eternal winter. The sister and Prince walk to the altar and proceed with the wedding, FL wishes her sister a happy marriage. But the sister isnt happy, in fact she ask the prince to stop the wedding as she wants to mary another, naturaly the prince donst take kindly to this. Sister gets angry and releases more potent controle magic and makes all tranched exept the ppl with Winter magic. The sister proceeds to explain that she will marry the man she was ment to aka Ml, that way she will have it all!. Ml prompty slaps her and away and says "No", the sister is in shock as he dosnt do her bidding.
Ml uses his Winter magic and removes the controle magic, leaving everyone to remember what the sister had said and done. The prince isnt happy with the sister, FL proptly tells her sister to stay away form her husband.
SIster screams at FL about getting in the way of her happyness, and FL answers with "at least i dont use magic to bend ppl to my will" and points at a red necklase the bad buy gave the sister (necklas enchances her controle magic).
Fl slaps her isster and tells her to stop being spoiled, all the guests start t whisper about the sister. The sister screams "unfair" and "why are you ruining everything", FL reminds her sister that she did all the work while the sister took all the credit. SO when FL married her work was reconised, where ass all the sister could do was spoit rightious words and throw a fit. FL mentiones that as she worked with her husband their love grew, and reminds the sister that she only married her husband because the sister threatened to kill herself if she had to marry him and yet here she is claiming FL "Stole" Ml.
The sister throws anotehr fit and starts to use controel magic, but FL swiftly takes the enhancing necklas. The sister beggs for it back, but FL uses Winter magic and destroys it.
The sister smiles and faints while the power of the badguy surges into Fl and goes amoch, ppl strugle to try and leave and the bad guy makes his entrance.
Sister wakes up and sees Badguy whom she calls out to, but he ignores her and walks to the center of the room and starts casting magic (he is trying to corrupt the Gral).
FL's body was the Key to opening a gate to the other side, meanwhile Ml remembers documentation pased down in the Winter household of how their land was cursed (human sacrifice was needed for the curse to be bound, hench the guests).

The Winter knighst fight to stop the deaths of citicents meanwhile, outside snow has
started to fall.
The civilians group together in a corner, while the Winter knighst and Ml protects them. The sister questions how all of this could happen, and her fatehr walks over to her and screams at her "You DID THIS!, WHAT WERE YOU THINKING!", the other civilians also start to wisper in unhappy tones bout her.
The sister screams back at them all, arousing the badguys intrest. Badguy walks over and seals all the civilians in a dark barrier, the sister runs up to him and says "this isnt what you promised" (or something of that sort). Badguy reminds her that he never made any promisess but offers her a hand! Badguy says she can have everything back if she drinks from his chalis (like her ansestor did when amking a pact), the sister promptly agree and drink it inspite of ther fatehr telling her not to.
The badguys power sourges in the sister who smiles and says "i feel the power", but it was short lived as the sisters body is incompatible hench she caughs up blood. Confused the sister saks whats going on, and badguy tells her she just wanst compatible and that she should have know that!. Badguy proceds to walk away from sister laying corrupted and bleeding on the ground, sister reaches for her mother and baby brother and asks for help ,but both mother and baby brother looks at her with fear and disgust while hugging eacother. Even the father cant look at the sister anymore and averts his gaze.

Badguy proseds to walk twards the unconciou FL in the Ml's arms, Ml protects FL with his own body.

A sceen is show of other ppl, presuably the Winterborns and how it all started. Duno to be honest.
To put it in simple terms.. FL accepts and becomes one with her Winterborn magic and wakes up.

FLs winterborn magic awakens and supressed the badguy, FL then proceds to gather and absorb all the badguys magic into her self and either seal, purify or destroy it.
Ml finishes off badguy, and the gate close and eternal winter is prevented.
Time passes and FL and her friends disguss the incident and FL's sisters punishment as she betrayed not only her family but the kingdome itself. Fl and Ml find them self happy, meanwhile the kingdome is still trying to fix itself. The prince is trying to apease all the nobels whos trust he has lost, while also strugeling with his own emotions and questioning why he ever loved the spoiled while sister of Fl.

Some more happy moments happen, and Ml shows Fl just how much he loves her. He searched and tracked down the locket her sister had stolen and thrown away!

Also The sister is miserable :p
Fl's spoiled sister got her wish :p she is now a commoner :p
Living under constant watch, having to work and is not respected.
She stays in a work house with and owner who is angry with her for her lack of getting up in the morning. The owner kicks her out and tells her to go do her job, if you don work you dont eat and so forth. The sister is now dirty, wearing an old ragged dress and only one sho, as the owner threw her out to get her to go to work. She quickly notices how everyone is glaring at her and wispering and feels looked down upon.

Cut to some time later, the sisters ex-bestfriend (the girl who could no longer stand er double standards) is going to a meeting ins a gorgeous dress and escorted by a gentelman. They enter the place they are going and a servant girl spills a bucket, the servant gilr is FL's sister with shoulderlength firty hair (had to sell for cash) dressed in rags mopping the floor.
The owner of the building scolds the sister for causing a sceen with guests there. The ex-friend reconises her and aproaches while her escort wonders why she would aproach "that".
The Ex.friend invites the sister for some tea, asks how she has been and so forth. The sister explains its hard, shes cold and strugeling and everyone is mean. The ex-friend says something along the line of thats the life of a commoner. That she hated on a loving sister and mistreated a nice fiance, that FL had done so much for her yet she never walued it and instead hated on her for it.
The sister asks what to do now or how to fix it and the ex friend prety much tells her that ship has sailed and to she needs to live with her choices shes made and their conciqueses (as she walsk away form the sister).
The sister cries and remmebers being in bed recovering with the prince next to her, she reached out her hand and asked him to look at her and he looked at her wit a blan dead stare while asking her "why?, wasnt i enough?". The sister cries again.

The rest is happy after some years chapters :p