Kimi to Boku no Kowareta Sekai

The first installment of the Sekai Series.
Two siblings in the process of crossing a forbidden line, Hitsuuchi Samatoki and Hitsuuchi Yorotsuki. Two friends, Mukaezuki Hakohiko and Kotohara Ririsu. A locked-room murder at the school cracks their worlds…
(Source: Kodansha, translated)