Karizuma - Kon'ya, Aniyome to Nemasu -

The protagonist is a fat and not-so-good-looking man called Mitsugu Yazaki.
He has an inferiority complex because of his brother's all-around greatness in professionality and looks.
Because of that, he struggles with socializing, and becomes a shut-in. His specialty is beating games. His lover is his 2D girls through his right hand. He is ultimately living his life as a NEET.
His investment program created with games, however proves successful.
As a result, he manages to leave home and live at a apartment, enjoying his life as a shut-in.
One day, his sister-in-law, Sayuri contacts him regarding his brother needing money to remodel his house.
In reality, his brother needing to remodel his house was just a front. His brother has actually amassed quite a lot of debt from his assets that have gone downhill.

Not only does the protagonist not feel obligated nor sympathetic for his brother, he also has an immoral desire for Sayuri.
Unbeknownst to his brother, he suggests that he can lend some money to Sayuri on one condition―to have a Karizuma (Wife-borrowing) contract.
This contract would let him ask for anything besides actual intercourse.
Busying around raising money, the protagonist is with his sister-in-law and new partner at home where his brother is nowhere to be seen.
Lust smeared all over, and curtains opened wide, this is where the first act begins.

[Translated from Getchu]