俺に彼女ができてから妹たちの様子がおかしい！？（Ore ni Kanojo ga Dekite kara Imouto-tachi no Yousu ga Okashii!?）

The protagonist, Amari Yuuta, lost his parents in an accident and now lives happily together with his three younger sisters while working as a pastry chef.

Recently, he had found a girlfriend at work and is now enjoying a steady relationship, but at the same time, his sisters begin acting strange around him...
The amount of risque outfits and excessive skinship from them is increasing.

"As their older brother, I need to tell them off..."

Yuuta was thinking about that when his sisters' actions became bolder and bolder.
It's the beginning of the days when he's constantly being pushed around by his sisters, both as an older brother, and as a man.

[Edited from DLsite]