I think that Dohwa told Yunwoo how to tell Hyewon and Yoosun apart when she spills their secret being 'twins' before thats why Yunwoo said that she can't see Hyewon so she removes her glasses (based on the side note) and took her chance to confirm her theory about one of them being a flower maiden. Correct me if I'm wrong or misinterpreted it.
I think that Dohwa told Yunwoo how to tell Hyewon and Yoosun apart when she spills their secret being 'twins' before thats why Yunwoo said that she can't see Hyewon so she removes her glasses (based on the side note) and took her chance to confirm her theory about one of them being a flower maiden. Correct me if I'm wrong or misinterpreted it.