Shimai to Now.

The protagonist has two sisters, who work as a model and an idol. To lighten their burden of supporting the family, he handles all the housework—cooking, laundry, cleaning. But as a growing boy, he harbors desires for his alluring siblings. One day, while masturbating to their swimsuit poster, his older sister catches him. Instead of scolding, she smiles and offers to help. Thus, they cross the line of brother and sister. Soon, his younger sister, sensing something amiss, also begins making advances.