My Stepmom's Daughter Is My Ex: Volume 3 by Kyosuke Kamishiro

Yo. I’m Kogure Kawanami, and my life’s a living hell ’cause I live next door to my psycho ex. She may look small and innocent, but she’s got more than a few screws loose. Right now, she’s totally obsessed with my friend’s stepsister, Yume Irido.

I thought she wanted to become Yume’s sister-in-law, but now she’s tryin’ to hook up some busty chick with Irido! Doesn’t matter, ’cause it’s not gonna work anyway! The Irido siblings have a strong, unbreakable bond. Trust me—my shipping radar never fails. They won’t end up like us. True love will prevail!