Gimai Seikatsu

From Yen Press:
When high schooler Yuuta Asamura learns his father plans to remarry, he’s not too bothered by the idea until he finds out his future stepmom has a daughter! Her name is Saki, and to Yuuta’s surprise, she’s a beauty who’s made a reputation for herself as an unapproachable “bad girl.” But as he gets to know her, he realizes she’s not the person everyone says she is. Could he be the one she finally opens her heart to?

Original Web Comic