Windows恋愛アドベンチャー
Main character is born as the eldest son of the left minister during Heian era, so he can spent days in laziness freely. One day father comes home with bad news - his only daughter is called to the palace for a month. In order to keep twin sister out of temptations of the capital, protagonist dresses as a girl and goes in her stead.
[From vndbreview]
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