Kyonyuu Onsenkyou Kodakara no Yu ~Densetsu no Hitou de Matte Ita no wa Bijin Oyako Deshita~

The protagonist is asked by a distant relative to help out at a hot-spring inn​ during the summer, but it's located at a secret place deep inside the mountains. After much trouble, he finally arrives, and await him are a pair of busty mother and daughter. Thus, his steamy days with them begins.

[Translated from Getchu]