Marie initially tells Celia she and Pearl will have to leave but then allows them to stay. What do you feel causes her change of heart? Did her decision surprise you?
I think deep down Marie Ah-Yi - and way later “Auntie Marie” - loved children and became attached to Pearl. Marie’s feelings for Celia grew stronger when she learned about Celia’s father.
I agree with you. The women at the brothel were a “family.” Marie is the head of the brothel.
I don’t know if you read Briar Club by Kate Quinn, but it’s another good example of women forming a “family” when they live together. They look out for each other.
The brothel contained a group of women that needed to look out and care for one another as a group of outcasts. Marie might have had a child herself that she gave up.
Agree with others! Also Celia was “help” unlike the other ladies. Her child, Pearl, wouldn’t affect the evening’s earnings. 
As others have pointed out, Marie was not really mean-spirited. One wouldn’t expect a child in a brothel—it would set a precedent for others to want the same. Certainly when Marie discovered Celia’s father was Chinese her opinion changed. She also realized Celia’s and Pearl’s total desperation. I liked Marie’s ability to show her compassionate side.
I thought Marie had a soft spot for Celia. Marie probably had a rough road when she was younger, too.
Marie had a sense of family in the brothel and I think realized that Celia was trying to protect her daughter.
Marie’s decision did surprise me. I was glad to see that the harsh exterior was for business matters, and that she had a heart for personal matters.
Marie revealed herself to be a woman who truly cared about others, despite the first impression McMorris gave readers of her. Again, McMorris deftly transformed the character as the story progressed, illustrating that she was also a victim of her circumstances. She loved children and protected Pearl, ultimately becoming Auntie Marie. I’m not convinced she ever really intended to make Celia and Pearl leave, but said that to ensure that Celia complied with the rules of the house. After all, Marie had to answer to the “big boss,” who turned out to be the evil Humphrey, so she had very little power. She was basically an enforcer.