Until the Unfortunate Tomboy Becomes Happy (?)

The Sarashina sisters Ibuki and Nagi were abandoned by their mother, and were about to be sent to separate orphanages. A wealthy middle aged pervert appeared before the two abandoned girls and he made them an offer, he would take care of both of them only if they became his sex toys. Ibuki accepted this unthinkable offer in order to not be separated of her little sister, on the condition that she alone would be the man's sex toy.