Koneko-chan, Kocchi ni Oide manga summary:

From MangaHelpers: After Yuko graduated from junior college, she ended up running a shop. Though her career is going well, she hasn't had time to work on her love life. One day, she's out drinking with her best friend Mikan, bemoaning her lack of a boyfriend for a Christmas date. When she trips and hurts her leg, Mikan asks her little brother Leo (an 18-year-old) to see Yuko home safely. When they arrive at her apartment, though, Yuko makes a move on him and they end up sleeping together. The next morning, she realizes that she not only slept with her best friend's little brother, she actually took his virginity! Leo isn't satisfied with it being a one-night stand, and they start having a completely sexual relationship. Will she be able to keep this from Mikan, and is it possible their relationship may grow to something more? [tethysdust]