
Nah it's really because she saw Yule (spoiler alert) was acting more like a normal person by doing stuff for Kyungsoo like washing dishes and "caring" for him and so she doesn't want Kyungsoo to leave Yule because he'll start acting up again

She never took off her shirt you probably got confused on one scene that is similar. when Yule was washing Kyungsoo and she was holding him and was blushing because of his moans and said "Kyungsoos body is so beautiful and his face is cute,where did you find someone as cute as him?"
Its seems like the sister genuinely cares about the blondie....might be wrong though