Gibo no Milk wa Mitsu no Aji ~Ippai Nonde, Amaete ne~

The main character, Yuuki Okazaki, is a college student who fells starved for affection.
He lives with his step mother and his father is away for the business.
One day, Yuuki could not resist the temptation to drink her mother milk.
And the scene was spotted out by her!
That is the beginning of their forbidden sex life...

[From DLsite English]