A twisted love triangle where two brothers fight over their childhood friend, the female protagonist. 4i3r3y

Even after the female protagonist relationship with Yuuki is found out, she continues her intimacy with Aki.
And as the days by, so too do his deviant demands of her escalate.
One day, Aki teases her with sex toys and leaves her in her room to endure the pleasure while he attends school.

However, worried that Mei hasn't been coming to school recently, Yuuki decides to pay her a visit... and thus witnesses her unbecoming display.

The indecent sight of Mei wriggling and writhing while stimulated by sex toys should be maddening, and yet, Yuuki cannot withhold his desires for her. He reaches out to touch Mei and...

After which, Aki returns home and bears witness to the two in the throes of ion... only to say:
"Mei, that's not good enough. You must sway your hips in a way that pleases men more."

And thus, the three fall... endlessly in the depths of debauchery------

There is a TL and R18 version.