Responses

Same. If you think is bad.
Toware no Kusabi - Ochita Reijo wa Bed de kawareru is 1000x worse.
The girl in that series ends up with the bodyguard who did nothing while repeatily gang raped by countless men and prostituted by his brother to cover for her cowardly father's debt who ran away and let her be whored out despite only being 19.
......
....wait for it...
The reason I think the stepbrother wanted her to "seduce" the 2nd ML was because 2nd ML's father (who I think is step bro's uncle) murdered the step bro's father and almost killed him and his mother while she was pregnant with the 2 or 3rd brother? can't tell which one. And he blackmails the FL that if she doesn't fulfil this task he will not pay for the FL uncle's medical bills.
And he makes her do various explicit sexual acts (except actual intercourse) to herself and on him to "practice". And treats her in a degrading way for quite some time.
Either way, half way through the story he kind of regrets sending her for the "task" since he develops romantic feelings for her -although it's implied he had a crush on her when he was younger but acted like Tsundere.
at some point while on a trip they end up having sex (he takes her virginity). But she leaves him and chooses to sleep with the 2nd ML in revenge against the step brother for the treatment she got from him and his family.
Seriously, they treated her like shit especially the stepmother. And when she went to her father as a child . He confronted the stepmother, step mom put on the innocent act and the father was instantly swayed.
The only brother that doesn't treat her like garbage is the other black hair one.
Even as a child he was the nicest.
For a time it seems her and the yellow hair guy have a sweet relationship and the FL appears to care for him and is happy. Since he treats her more tenderly than her pimp brother.
And he takes care of the expenses of her uncle's bills so she thinks she is finally "free" of her past torment she experienced - she is wrong. The "nice guy" shows his true colours and ends up being worse than step bro.
You guessed it.
She ends up with the PIMP older stepbrother stepbrother. Although, for a time she chooses the other ML whom for most of the story seemed liked the better choice.
Until it is revealed he suffers from a deep rooted inferiority complex that her step brother only intensifies further the more he gets involved in business affairs.
Due to the difference of their abilities being obvious for others to see.
He takes out his frustration on the FL. Raping and beating her up frequently. I think he also has become aware of her past liaison with her step bro which makes it worse.
That she is covered in wounds and bruises and gets locked up in their home.
And photos of her battered and bruised get sent to the step brother. Step bro is horrified.
Fast forward some events...
With the help of the the red haired bitch she tricks the yellow guy on their wedding day and switches places with the red hair bitch whom was the yellow hair guy original fiancée.
And she escapes to find the pimp stepbrother and they settle in the place where they first had sex.
I'm spoiling now to save people time if they believe this is enough for them to drop it.
Personally, I think the only reason the author made the yellow hair guy have a complete 180 personality change was to justify her ending up with the step brother. And make him seem the "better" choice. Which doesn't fly imo.