Acerola - Sorceress Tia and the Great Library of Wonder Version 1.0 (jap) - Adult Sex Game

The tale unfolds in a world where magic is common and accepted in everyday life.
Tia and Lumielle are twins, and descendants of a great sorcerer.
The two run a clinic in a remote town called Almeria.

One day while they are eating, they hear a strange sound from the clinic basement.
It's coming from their ancestor's archive in the basement...
Thinking it may be a ghost, the two go to check things out.
There, they see a tome sealed with strange light.
When Tia tries to touch the tome, the seal is broken, and the ancient book opens by itself...

From the book springs a mist and a succubus named Elaine,
who promptly defeats them, takes their virginity, and brands them with a lewd mark.
Finally, Lumielle is sucked into the book....

"If you can find your sister, I'll return her to you.
However, if you don't start having lots of sex, something bad will happen to her..."

It's up to Tia to search for her beloved sister, while trying not to drown in a sea of pleasure.