What manga is this?

I can't remember the name of this manga. It has a step brother and sister who don't really get along. One day the college age sister had a friend who dressed her up in a high school uniform, and had her go walk around. She ran into her brother (also college age), but he didn't recognize her. He ended up having love at first sight or something. They end up dating. Funny stuff ensues. She starts to feel guilty and tries to break it off. He breaks down. She keeps dating him, so she can repair their brother sister relationship, so he'll have some support the next time she breaks up with him. Anybody know this one?