Kojire ni Kojireta Gikyoudai no Hanashi

Rikka, a working woman, has a young stepbrother Rin, a college student. For over ten years, Rin has secretly harbored feelings for his beautiful and responsible stepsister. He was convinced that she only saw him as a little brother and that he could never confess his feelings… until they end up going out on a “practice date”!?
For the first time, he's calling her by her first name, holding hands together… As they act more and more like a real couple, his pent-up love for her is on the verge of bursting!?

(Source: Gangan Online, translated)