Aniyome ni Tanezuke Haramase -Gishi wa Ore no Kodane o Kobamenai- by Appetite - Adult Sex Game

Takeuchi brothers and [Risa], who are neighbors, have been friends for a long time.
As a matter of course, the Takeuchi brothers liked her.

Eventually, Risa married her brother, Seiichi.
Her younger brother, Ryuji, was superficially pleased, but was accumulating inferiority complex and anger at her brother.

Seiichi, a perfect human being, has never experienced a setback.
If he experiences the first setback in his life at this age, he may not be able to recover.

So Ryuji proposes to Risa.

"I'll conceive her sister-in-law instead of my big brother."