Part One: Callie wanted Stef and Lena to be her mothers more than anything, but there was still a huge problem. She was in love with Brandon and if she'd get adopted, they wouldn't be able to be together. One thing led to another, they spent a night alone, things just happened. Callie didn't expect to later find out she was pregnant with her foster sibling's baby.
Part Two: At the age of 19 and 20, Brandon and Callie both weren't expecting to have a four year old son or have their daughter on the way. They didn't think they'd be living in a house together - let alone be married for almost 5 years. They've been through miscarriages, accidents, relationship problems - but they've only come out stronger in it all. They were ready to move on from the past and leave it all behind. Sometimes, things don't always work out how they want it to.
Part three: "I played piano and you played guitar. We both created something beautiful but in the end, strings break and keys lose their melody."