Days with My Stepsister Novel Volume 3 was written by Ghost Mikawa with illustrations by Hiten.

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?