Houkago Rinkan Chuudoku

You have transferred to the same school as your younger step sister, Sayo.
But, Sayo has fallen victim to a dastardly trap set by the school's queen bee Elena and is now a raving sex addict. Will you attempt to save Sayo, or perhaps lend a helping hand in her descent into debauchery?
A secret after school stage of sex, service and gangbangery.

[From DLsite]