Stealing My Brother's Wife - The Lewd Secret of My Sister-in-Law, Melted by My Blackmail

My estranged older brother has recently returned to our home. And he's brought his beautiful wife with him.
He says he'll only be there until he finds a new job, but will that really be the case?

This work tells the story of the protagonist’s wife and her younger sister, who get a job at a gym and gradually fall into depravity.

During the summer vacation when his parents are away, he begins living with his step-sister.

Back in school, your teacher once dubbed you a "Maltese Tiger"—a rare, mythical beast bursting with untamed potential.
But potential doesn’t pay the bills. The real world hit hard, and graduation left you dead broke and jobless. Desperate, you relied on your mother—a lifelong housekeeper for the wealthy Lee family—to secure you a bottom-rung job at the Lee Group’s real estate empire. The man running the show? Shane Lee. The golden-boy heir you grew up alongside. A man you once called a brother.
When your mother fell critically ill, you went to Shane for help. Instead of a lifeline, he threw you a leash.
With cold disdain, he ordered you to move into the estate and act as a glorified dog-sitter, banishing you to the same servant's quarters your mother used to occupy. His parting words before leaving on a month-long business trip? Wait there, and know your place.
It was only when you arrived at the sprawling Lee mansion that you discovered the sickening truth. Your mother’s "room" was nothing but a cage—a suffocating, five-foot-high utility closet where a grown man couldn't even stand up straight.
But Shane forgot one crucial detail: lock a wild beast in a cage, and it doesn't become a tame dog. It just gets hungry.
You have exactly one month.
Starting from this humiliating dungeon, you're going to claw your way up the manor's hierarchy—from the ground floor to the master suites. You will make your way into the bedrooms of the women Shane holds dearest: his sister, his wife, and his mother. You're going to use your cock to conquer his closest kin, dismantle his pride, and steal his entire life for yourself.

Due to parental circumstances, a brother and sister end up living together for 30 days.
The sister has hit adolescence, and although it's been a while since they last lived together, she seems somewhat aloof.
The two talk, interact... and sometimes even play naughty pranks.
Alone in a room, they gradually grow closer...
A lovey-dovey little sister cohabitation simulation!