Liar Taisen Janmagedon

Main character Ueda installs new Liar Soft CD to the pc in the living room while his sister secretly watches him from a distance. Suddenly girl introducing as a new heroine appears on the screen asking for help in their fight against a nasty beast. The monster threatens to turn a pure love game into tentacles rape feast. As Ueda agrees to help, both he and his sister get transferred to the Liar Soft world.

[From vndbreview]