[MON-MON] My Kind Sister's Dramatic Transformation (Koyoi, Tsuma ga Sarasarete…) [nonameneko]

A kind office-worker sister starts coming home later and later, her modest blouses swapped for increasingly scandalous outfits as a man named Yuuichi quietly remakes her into his obedient plaything. Her younger brother Kouhei can only watch in mounting dread as Kiwomi trades family dinners for ball gags, nipple clamps, and latex, ultimately kneeling in a collar and cuffs and begging her master to take her front or back. Told largely through the brother's helpless eyes, this is a slow-burn corruption story where every shuddering, gagged moan marks another step in the dramatic transformation of the sister he thought he knew.