Rina no Chiisana Oka

Rina lives with her mother and brother.
Her mother's remarriage triggers her to run away to her brother-in-law Kyosuke's apartment, whom she had a secret crush on when she was little.
When Kyosuke asks her to have sexual relations with him, Rina is at a loss, but then she discovers a secret about her friend Mai.
The two have not yet grown up physically as they have mentally.
The summer vacation is approaching, and they are still keeping secrets from each other.