This is my own summary of the story. The story is about Yuusei (son), who is in love with his own mother (Kyouko) and wants to have sex but is afraid and can't express his own feelings. However, Kyouko's older sister (Mayumi), Yuusei's aunt, decides to help him. To do that, Mayumi will dress and pretend to be Yuusei's mother and teach Yuusei how to have sex. Kyouko does not know that something like that is happening.
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The more you read, the more you will notice something. Kyouko is not Yuusei's mother. It's actually his aunt. Mayumi is actually Yuusei's mother and not his aunt. The father is married to Mayumi but fell in love with Kyouko, so he divorced Mayumi and remarried Kyouko. Kyouko birthed a daughter, while Mayumi birthed a son who is Yuusei. Understand now? The story is interesting. The father died of an illness after both women gave birth. Damn, father or maybe not. This lead to both sisters being unable to have a nice relationship. The young sister Kyouko decides to raise the son instead of the daughter. The baby was swapped. It was kept a secret from the children even at the end of the manga. Only the reader knows the secrets. Almost at the end, Yuusei was able to have lots of sex with Kyouko and express his feeling. Kyouko also feels the same and hopes to live together forever. Also, Mayumi is pregnant with Yuusei's child. He doesn't know that.
