I recommend the novel "Seeing You Knows the Feeling of Spring" to you. The female protagonist, Meng Hailan, is a Taekwondo girl and also the teacher of the male protagonist, Chu Jingzhe. Although he was not the male protagonist's younger brother, the male protagonist was very sweet to the female protagonist. The two of them had a warm teacher-student relationship. I hope you like my recommendation. Muah ~