Need help finding a motivation

[samisit]

So basically the background story is:
Woman gets pregnant through prostitution; gives child to friends to build a brothel and become a business woman herself; child befriends foster brother who takes care of her; woman is successful, but after roughly 10 years wants her child back for own purposes in brothel; foster family are human traffickers themselves and want to keep the child; woman has them killed except foster brother who hands her the child eventually.
What could be the motivation of the foster brother to go to the brothel and see the protagonist? He is essential to the later plot, because he is later asked by the protagonist (child) to help her buy another young girl that is being auctioned to set her free. Since she doesn't know anybody, who can help her, she asks the one she once trusted. In the end however, he betrays the protagonist (for whatever reason) and wants to keep the child, which leads to negotiations between them.
Why would he go to the brothel when he is aware of the fact, that the brothel owner would throw him out because of their shared past?
Why would he betray the protagonist in the end?

Has someone an idea for that?

Edit:
Okay, my bad, I wasn't really clear about the plot and the characters and rushed through the post, so:

The Protagonist (A) is a young woman who is forced into sex labour in a brothel. She and other women have different mechanisms to go through that harsh time, but they all try to cope. The protagonist has grown up in a foster family before she came to the brothel. She was put there by her mother who was more focused on building the brothel than caring for her child. The only caring person there was his older foster brother, who looked after her and was the only person she had a remote connection with. When her mother wants her daughter back, the family demands money for all the grooming etc.. Since she needs to live out her control complex, she buys her own daughter. A comes into the brothel and is forced to have things happen to her against her will. Almost a decade later A is a complete wreck and miserable, traumatized by the things that happened throughout her life there. One day, she tries to end her life, but changes her mind as another young girl enters the brothel. A is reminded of her horrible childhood and feels the responsibility to prevent history from repeating itself. Many things happen and she decides to rescue her and bring her out of the brothel. She becomes the right hand of the owner of the brothel and even though she now has to punish and force her former colleagues to sex, she uses her position to initiate a rescue mission for the younger girl. This is where her former foster brother comes into play. She reads his name on the list for the attendees at an upcoming auction where the younger girl is also being auctioned. Even though she doesn't trust him anymore, since she thinks he sold her to her own mother/ to the brothel when she was still with the foster family, she asks him if he would buy the little girl and then bring her home. He agrees, but in the end he thinks that now that he did this for A, she would finally open up to him and even love him, because he has felt love for her all along. Things escalate, A kills her own mother after a long conversation and takes control of the brothel, giving her the opportunity to negotiate with her foster brother, who claims ownership of the young girl. A, completely desperate, offers one of her colleagues in exchange for the younger girl. The foster brother enjoys to see how his little girl has grown to a human trafficker herself and complies, looking forward to have her now as a trafficker partner.

I hope, this clears things up a bit. So her foster brother is kind of attracted to A, which is also why he initially helps her and is disappointed in the end, when he learns, she just wanted to rescue that younger girl. What I still don't know is, why the foster brother would come after all these years back to the brothel to see A, who has aged almost 10 years. I thought that JUST business reasons would be a little too poor. What can be a realistic goal?
greetings
sam

[Sir-KP]

Why did you want him to betray her in the first place?

[ArrowInTheBowOfTheLord]

Your description of the plot is sort of convoluted, so I'm having trouble understanding the threads. I'm not really picking up on the motivations of any of the character actions.

[BornForBurning]

Why would he go to the brothel when he is aware of the fact, that the brothel owner would throw him out because of their shared past?

Because he cares about her.

Why would he betray the protagonist in the end?

Because he wants to have the girl who is being auctioned as his sex slave. So there's tension between him as a good guy (wants to help foster sister) and a bad guy (perverse sexual desires). Ultimately, his bad guy side would win. If I was writing something this horribly miserable, that's probably what I would go with.

[samisit]

Okay, my bad, I wasn't really clear about the plot and the characters and rushed through the post, so:

The Protagonist (A) is a young woman who is forced into sex labour in a brothel. She and other women have different mechanisms to go through that harsh time, but they all try to cope. The protagonist has grown up in a foster family before she came to the brothel. She was put there by her mother who was more focused on building the brothel than caring for her child. The only caring person there was his older foster brother, who looked after her and was the only person she had a remote connection with. When her mother wants her daughter back, the family demands money for all the grooming etc.. Since she needs to live out her control complex, she buys her own daughter. A comes into the brothel and is forced to have things happen to her against her will. Almost a decade later A is a complete wreck and miserable, traumatized by the things that happened throughout her life there. One day, she tries to end her life, but changes her mind as another young girl enters the brothel. A is reminded of her horrible childhood and feels the responsibility to prevent history from repeating itself. Many things happen and she decides to rescue her and bring her out of the brothel. She becomes the right hand of the owner of the brothel and even though she now has to punish and force her former colleagues to sex, she uses her position to initiate a rescue mission for the younger girl. This is where her former foster brother comes into play. She reads his name on the list for the attendees at an upcoming auction where the younger girl is also being auctioned. Even though she doesn't trust him anymore, since she thinks he sold her to her own mother/ to the brothel when she was still with the foster family, she asks him if he would buy the little girl and then bring her home. He agrees, but in the end he thinks that now that he did this for A, she would finally open up to him and even love him, because he has felt love for her all along. Things escalate, A kills her own mother after a long conversation and takes control of the brothel, giving her the opportunity to negotiate with her foster brother, who claims ownership of the young girl. A, completely desperate, offers one of her colleagues in exchange for the younger girl. The foster brother enjoys to see how his little girl has grown to a human trafficker herself and complies, looking forward to have her now as a trafficker partner.

I hope, this clears things up a bit. So her foster brother is kind of attracted to A, which is also why he initially helps her and is disappointed in the end, when he learns, she just wanted to rescue that younger girl. What I still don't know is, why the foster brother would come after all these years back to the brothel to see A, who has aged almost 10 years. I thought that JUST business reasons would be a little too poor. What can be a realistic goal?

[Ralph Rotten]

Honestly, if she was a captive in a brothel, they would never have let her go full term with the baby in the first place. These places are all about making money, and a pregnant woman will not be able to bring in as much money, so that pregnancy would be terminated. I think you have a fanciful ideal of how things work in brothels and the sexploitation trade.

[samisit]

That's obviously a misunderstanding or not stated explicitly enough on my part. The woman who got pregnant through prostitution is the one who later builds the brothel. She was engaged in prostitution on the street, until she got pregnant through rape and had to quit working as a prostitute due to her state. That's when she took matters into her own hands, used the influence and money she had and built a brothel. And that child she bears is the one that is referred as A in the summary. The owner of the brothel is the mother of the protagonist aka later becoming one of her prostitutes. I very well know the dynamics of this matter.

[InkVains]

Hmm, I think more elaboration on the whole plot would help me understand this better. I don't think it's a bad concept but a little more detail added to the relationships between the characters would personally be more interesting to me.