Hyakumankai Kiss Shitai!

From MangaHelpers:

Yu is a college drop out who works part time, but he's at his happiest point in his life so far. He just started to become really intimate with his girlfriend Saya and they can’t keep their hands off each other. But Yu’s stepsister, Sana, suddenly pops into his life after 5 years. They were separated when their parents divorced and with their mother getting remarried, Sana could not stand living with her anymore. With nowhere else to go, Sana starts living together with Yu, but at 19 years old, she has grown into her body. Yu is torn between the two women in his life and has to pick one or risk losing both of them. [vyc]