Mama no Bonyuu

Our protagonist, Tachibana Yuya, lives with his stepmother, Kaori, who runs a small modeling agency.
Although Yuya is deeply attracted to Minako, who moved in next door six months ago, he lives his days peacefully without confessing his feelings.

One day, Yuya witnesses Kaori in the indecent act of masturbating, squirting breast milk during a climax.
Shocked, Yuya goes to the park, feeling distressed, but Minako, who happens to be passing by, invites him over for tea at her house.
Seeing Yuya struggling with his feelings for his mother, Minako gently embraces him.
"I'll never abandon you. Never again..."
Minako whispers this as she begins to service Yuya with her mouth...

[From Getchu]