The engagement was with the younger brother, but I've fallen for the older brother.

(1v1 between two virgins + mature man and woman, lust followed by love + 7-year age gap + older man/younger woman guiding type)
[Sweet Fairy Girl X Innocent Scoundrel]
After transmigrating into the body of the female protagonist in a tragic novel, her engagement was to her younger brother, she was to be with her older brother, and her bed was stolen.
To appease the scheming woman, her fiancé handed her over to his abstinent older brother for care.
This care extended to the bedside.
To outsiders, his brother is spotless and aloof, a doctor with few desires.
Only Si Yimian knew that He Sichen had been innocent his whole life, and all the dirt was on her.
He Sichen wanted to be a saint, but he wanted to be her person even more.
On the day the engagement was nullified, her fiancé broke all his defenses.
"She's my fiancée!"
He Sichen held her, his voice languid: "Not anymore."
"What do you take her for?"
He Sichen: "To be your sister-in-law."
"You personally brought her to me, begging me to take care of her."
-
He Sichen had never been a dog in his life until he was dumped by Si Yimian.
The usually aloof man knelt on the edge of the bed, gripping her wrist, his eyes bloodshot.
"You lured me down this twisted path, and now you're tired of it?"
"Si Yimian, even your attempts to manipulate people's hearts are all half-hearted."
"You used to say you wanted that hypocrite to kneel down and be your dog."
"I've gotten my license now, I'm a legal pet, please take me back."
Serving Guide:
She advances, he retreats; she retreats, he goes crazy.
The female lead is not a traditional innocent girl; she is a sophisticated egoist with an extremely high sense of self-worth.
On the surface, it's like a peach-shaped glutinous rice ball; in reality, it's a top-tier operator with flexible moral boundaries.
The male lead knows the female lead is selfish and being used, yet he still chooses to protect her and falls madly in love with her.