Days with My Stepsister, Vol. 3 (Manga)

It's Yuuta and Saki's first summer break as stepsiblings. Now that Saki has gotten a job at the bookstore where Yuuta works, they've ended up spending more time together, but she seems really stressed. When she even turns down an invitation to go to the pool with everyone, Yuuta decides to figure out what's going on and help her relax. In the process, he finds himself thinking of her in ways that a brother shouldn't...