Hajimete no Onii-chan

This guy is way too exciting and dangerous! The obsessive romance comedy begins♡

Kokoro is a new first-year high school student living with her single mother. When they go to meet her mother's new fiancé... who shows up is none other than her ex-boyfriend, Izumi Aoi-senpai! She thought she was the only one still thinking about him, but it turns out Aoi cherished their past memories too—he'd even kept a candy wrapper for two whole years. He seems... pretty intense!? Aoi declares, “Lovers part ways when they break up, but siblings can never truly sever their bond.”, leaving Kokoro to ponder... What will become of this forbidden step-sibling relationship between former lovers──!?

(Source: Shueisha, translated)