From Good Girls Scan BL:

Yukito is an average high school student with a gorgeous and outstanding twin sister, whose only similarity she shares with him is her face. Yukito, who's always getting compared to his sister, also has someone he likes - that person being his cousin, Taiki. There are rumors of the cool and popular Taiki and Yukito's sister liking each other. The relationship between Taiki and Yukito had become one where they'd both grown reserved around another, but using the preparations for the school festival as an opportunity, Yukito and Taiki both quickly grow close again! Yukito's heart wouldn't stop racing after being saved by Taiki when he was being troubled...! Moreover, in an unexpected turn of events, the two of them started sharing ""a secret""...?

The secret love between two cousins!