It was a lengthy hentai with multiple chapters. Main character is a dude who moves back to his hometown and hangs out with a pair of sisters who he was friends with back in high school. He makes a move when one is in the bathtub only to find that it's the wrong sister. Of course they fuck anyway.

From there it devolves into endless fucking as he works his way through the rest of his class. At one point one of the girls tricks him into fucking her with a sob story about how her LDR boyfriend never got her off but she has sexy lingerie, they fuck in the school pool, they fuck at the local swimming hole, etc. The series ends with an orgy where the main character drinks some shit with a snake or something in it. That's about all I can remember. It's been around for years.

Optional side mission: What's the song in this animation?

Thanks in advance.