@Neko-min, here is my take on the situation.
The mother was a highly valued courtesan that is able to earn her keep with her skills, not sex.
She agreed to have sex and have a baby with Rakan. Her value plummeted. But she should not be forced to have sex during this time.
She have our MC. Then she must earn her keep. This is when her plummeted values get real. At the beginning, she might only agree to get on the old same road aka sell skills not sex but no customers would mean no income.
And she was the focal women of this place. Her lost reputation would certainly mean a lost in revenue and customers. This would affect others women in the establishment as well.
Then she has no other choice as Rakan has broken his promise of redeeming her. She has no income, no nothing. I can easily see that she has to agree to sell sex from then on. Remember that Granny patient can only go so long and as a de facto boss of the place, she cannot keep a non-earner after maybe 1 year since she started her pregnancy.
And the guests start to come and you can see it was a slide from the slope from the richer guys, then middle class guys, and downwards. She can only choose so much if no one comes to her. And as we can see from the story, she contracted the disease. And such disease is vicious on a body of a courtesan.
Please think of postpartum depression which she were really likely to have as a stress of having a baby, earning her keep and the realization of a broken promise, then her own pride, earning to feed both herself and her child then the realization that she contracted the disease. That is by no mean an easy feat for any young person. And with the disease got worse so was her mental health deterioration. At one point, probably after recalling her broken promise in a mental break down episode, she cut off the finger as in the story and gave our MC the trauma.
Think of the Granny. She was probably a courtesan once seeing all of those come, go, ascent or perish. She must have had taught rules into her girls and one of the biggest if not the biggest is to never have sex before the buy back. This rule benefits both the women and the master of the place at the same time in case of a broken promise. And no, this is not a strange or rare tale for the likes of broken promise in the Sinospheric countries. Songs, poems, books from those times has records on those instances. And the mother must have known this and taken upon herself an unnecessary risk due to love. She believed Rakan that much that everything was placed on the line just for some words from a dreamy boy with no money, no power, nothing to his name or his own.
She failed her bet. She lost it all. She almost killed her child. Now she was an empty shell of her past. I do not condemn her but feel sadness for such life instead.
The Granny did not throw her out on the street. She kept her even Rakan has not come back. She must have seen people like this mother but what are the options for her to do? She allow this mother to live in a separated place and some basic things for her. No the Granny is not the second coming of Christ but she does display some sympathy towards her former employee. Her beef with Rakan probably also from her personal feelings as well. Rakan was still in the wrong, especially with the mother and he was the main reason for things to go this way. And no, the Granny cannot or should never give too much care for the mother as inhuman as it sounds. Think of all other women to do the same. It would basically bankrupt everyone and ended that establishment. Remember that Granny was probably a courtesan herself in the past? Yeah to rise to the rank and at a senior age, she probably harden her ways up to the tops. A cold, stoic boss who still shows some sympathy and little kindness to her fallen employee/ coworker.
Brothels are never a place to find a paragon of human characteristics but the most wretches ones instead. Granny, in her defence, is a stoic woman who can only act so much with the given situation.