Summer Memories ~Girls who were cuckolded~ Another story

Kasumi, a bright and energetic girl,

is childhood friends with Mamoru, the main character, and with her parents' permission, they live together during summer vacation.

Although she admires and is jealous of Mizuki, who is her identical twin sister and her older sister, she
suppresses her feelings for Mamoru and lives with her.
She thought this summer would go by as usual, but...