Pure Mail

School Days and Pure Mail take place in the same universe of 0verflow's visual novel franchise. Sawanaga Miki (Pure Mail) is a sister of Sawanaga Taisuke (School Days) canonically. The abusive father Sawagoe Tomaru had relations with several characters in the francise including incestuous ones. Both main male protagonists Ogata Kei (Pure Mail) and Itou Makoto (School Days) are his sons from different women. 

What do Pure Mail and Milkyway have in common? Well, for starters, they're both 2-episode hentai OVAs that came out in the early 2000s. But more importantly, they're both based on eroge that were made/worked on by 0verflow, so they actually share the same universe! Neat, huh? 

both have same school ecchi character 

True to life drama focusing on story and character.