Kanojo no Imouto to Kiss wo Shita. (Novel)

From Tentai Books: "We'd better not let my sis see us like this." My girlfriend's younger twin sister has somehow become my new stepsister. "You sound really nervous right now. You're just so adorable, bro." It's been a month since I started dating Haruka, my first-ever girlfriend. We even finally reached first base today - holding hands. "What a bad boyfriend you are. To think you're daydreaming about your little sister while you’re holding hands with your girlfriend, of all people!" Today's also special for a different reason my dad's getting remarried, which means I get a new stepsister. Her name is Shigure, and she's pretty much the spitting image of Haruka. That's no surprise though, given that she's actually Haruka's long-lost twin sister! Due to family circumstances, they were split up at birth. "It's okay, bro. I look exactly like sis does, right down to well, you know. I don't blame you at all." Her presence adds a new, twisted form of love that tangles itself into our protagonist's passionate love life! He can't possibly tell his girlfriend what's been going on.