Like the other relationships that Niko has with his friends in GTA IV, favor can be gained with girlfriends by taking them to different venues around the city.
After completing the mission Waste Not Want Knots, Patrick Mc Reary will call to suggest to Niko Bellic to take his sister on a date.
Dating these girls is not a requirement to attain the 100% game completion stat, however each has a special ability that is unlocked once they like you enough.
This feature is available after the “Out of the Closet” mission in Brucie's mission line.
Since Kate and Michelle are fairly easy to please and only pertain to the storyline, they won't be covered here.
There are three other girls that Niko can meet and date via the