Erik met Elena his sophomore year in College. He enjoyed her company. Not only did she help keep his mind off the trouble he was having with his dad (Dave) telling Erik that Dave was getting remarried but she was one of the nicest people Erik had met. A couple months after they started dating they started having sex.
Elena didn't share much about her past besides her mom and sister had just moved to a new town before she started school there and the fact that her dad had passed away four years prior to the story's beginning.
After a while Dave says that things are getting crazy. His now new wife is having troubles with her oldest daughter. After that things settle down between Erik and his dad enough that Erik tells his dad that he plans on proposing to Elena.
Just before Erik proposes Elena tells him she's pregnant. He proposes then and there and she says no thinking that he is only propsing because she is pregnant. Her parents got married because her mom got pregnant and they were never very happy in life. The only good thing in their marriage was their kids and sex. But when Erik bends to one knee and shows the ring and propses Elena realizes Erik's true feelings and the meaning behind the proposal and says yes.
At the end of the school year Erik and his new wife head to his home. On the way there Elena is getting more and more uneasy. Erik thinks it's because they're going to see his dad before meeting up with her mom. It isn't until after Elena sees the address of Erik's dad's house and sees a familiar car that they find out that they need to talk.
When they get out of the car Erik asks what's wrong. Before Elena can answer her sister steps out of the front door of Erik's childhood home. Shortly after that is when they both find out for sure that Erik's dad had married Elena's mom and that they are in fact step-siblings.