https://www.appetite-game.com/apt_172.html
http://www.getchu.com/soft.phtml?id=1102690

Story summary:
Wife's sister, Mimori, is seducing MC regularly, as the affair goes on MC realises he gradually lost interest in his wife.
Mimori comes up with a "solution": she dresses up as the wife and goes on a date with MC.

There are 2 routes.
Ending 1: During sex if MC proclaims his love to Mimori, then it is revealed the wife has been cheating on him for a very long time.
They end up divorcing and MC moves into a small flat with Mimori, who no longer dresses up as a gyaru and has become a "neat&clean" type of girl.
She admits she only dressed up as a gyaru, because it was easier for her to approach MC and claims it was a love at first sight.
At the end of the story MC remarries with Mimori.

Ending 2: if during sex MC claims he still loves his wife, then Mimori storms off, saying it's boring to pretend to be her big sister.
Next time Mimori starts to seduce MC again and they end up having sex. Wife opens the door on them and Mimori becomes crazy and laughs how she enjoys stealing MC from her sister, but once the deed is done she disappears from their life.
MC tries looking for her and some time later he finds her, but now she looks like a total slut, he tries to apologize to her after realizing she did loved him, but Mimori says it's too late and disappears into the night with her (sex) partners.

Base +6