Otoko Benjo ~Imouto Mitai na Otouto to Otouto Mitai na Imouto o Ijimeru Natsuyasumi~

The protagonist is a university student living in Tokyo. During his final summer vacation, he returns home for the first time in years.
He expects to find his parents, younger brother, and younger sister there, but...
His younger brother has transformed into a beautiful girl in a white dress, and his younger sister into a lively boy in running shorts!

As his parents are at their wits' end over their strange behavior, the protagonist is reminded of a certain "game" he used to play with his younger siblings when he was little.
-- ...It must be because of that "switch game"... --
In the past, the protagonist used to dress his timid younger brother as a girl,
and his lively younger sister as a boy, playing pranks on them as he pleased.
It seems that this experience had an "impact" on them.

"Well, maybe I should tease them then?"

During the five days he's back home, he decides to play some old-fashioned pranks on the beautiful young brother and energetic young sister who adore him.
The protagonist's sadistic tendencies are resurrected in his heart...

[From Dlsite]