Don't Stop Mama?!

Main characters got divorced, and soon his father remarried.
12 years later. Father starts to cheat on his wife, and protagonist is very sorry for his stepmother. Stepmother acts in a sane manner, but there is no more cheerfulness in her smile. So protagonist could not refuse her invitation to take a bath together for the first time in a while. And this turned out to be his first mistake...

[From vndbreview]