Yes, Satsu is a prostitute and her sister Chiyo is shocked when she learns the truth.
One day Yoko sends Chiyo to the "Mizuki Teahouse." When she arrives there she learns to her shock that her sister has become a prostitute:"I can't possibly describe how I felt when I saw the sign lettered "Tatsuyo." She also learns that Satsu has now been rechristened as 'Yukiyo.' When Chiyo enquires for her sister she is toldthat "Yukiyo has a customer. When he's done someone will tell her to come down."
Chapter seven contains the poignant reunion of the two long separated sisters. Satsu is determined to escape from the brothel: We'll run away Tuesdeay late at night, five days from now. I have to go upstairs, Chiyo. A man has come for me."