Days with My Stepsister, Vol. 1 (manga)

After his father's remarriage, high school student Yuuta suddenly finds himself "stepsiblings" with his classmate Saki. However, unlike the plain and simple Yuuta, his new younger sister has an over-the-top sense of style and is known as the hottest girl in the grade! Due to past trauma, Yuuta isn't great at talking to women, and at first is perplexed by this new family member who seems to be his complete opposite...but as the two settle into a shared lifestyle, could their relationship blossom into something more?!