Akizuki-ke no Jusei Irai -Ninshin Suru made Kurikaesareru, Tsuma to Ani to no Sex-

"He was only supposed to be a surrogate."

The tests proved it: a would-be father couldn't bear children.

It was a bitter realization, but that was nothing compared to
watching his beloved wife f*ck his brother every day and fall in love.

"You don't both have to orgasm......
why do you keep going.......
why would you do this to me......"

[from DLSite English]