Days with My Stepsister, Vol. 3 (light novel)

It's Yuuta's and Saki's first summer vacation as stepsiblings, and they'll be working at the same bookstore! As her senior coworker, Yuuta begins to see a side of Saki he hadn't noticed before. But when Shiori works a shift with her, she makes a dark observation—if Saki doesn't ease up and start relying on others, sooner or later she's going to destroy herself. If he wants to help, Yuuta will have to make a decision. Should he betray his own rules and interfere in his stepsister's life?