My Stepmom's Daughter Is My Ex: Volume 4 by Kyosuke Kamishiro

Your first kiss is something you remember your entire life. The awkwardness, the embarrassment, the knowledge that things will never be the same between you again...and more than anything, the absolute bliss of that special moment. Despite how things turned out between us, our first kiss is burned into my mind.
Similarly, you never forget your first love. I know I haven’t. Mizuto was mine, but would he say the same about me? Surely he doesn’t think his first love is this extroverted college girl who has her act together, right? So why is he following her around like a little puppy?!
Argh, why do I even care?! We broke up. We’re stepsiblings now. There aren’t any lingering feelings between us. So how come I can’t stop thinking about our first kiss? Maybe if I’d been more assertive... No. There’s no reason to dwell on the past. I need to focus on what’s ahead, like our family trip to his father’s hometown...