Milk Teacher

Ayumu, a university student, has a crush on one of his teachers, Fumiko. He is very much enamoured with her huge breasts, but he also likes other things about her, such as her dedication. Unfortunately, not only is she married, she is also having an affair with her former student. As if that wasn't bad enough, the former student is Ayumu's older brother, Kakeru, and Fumiko is very devoted to him. For better or worse though, Kakeru is using Ayumu's room as a hotel for when he comes from Tokyo to have fun with Fumiko. Watching them flirt is painful for Ayumu, but at least it gives him the chance to get closer to Fumiko, especially since Kakeru is not at all stingy and is away much of the year. Fumiko's huge libido and wilingness to follow Kakeru's orders definitely help too.