Alex didn't have a great relationship with her sister growing up. It's not to so bad now, but a while back, her sister was dating Rob, and there was no secret that Alex had the biggest crush on him. Her sister would take every opportunity to rub it in Alex's face. When she bumps into him by surprise, all the old feelings came flooding back. He has done very well for himself, and after exchanging numbers, she ends up at his place for a few drinks. Now its her turn to make her sister jealous. Karma is indeed a bitch.