Dawn Summers isn't really anyone's love child. Sometimes siblings just appear out of nowhere and come pre-packaged with bogus memories. When the youngest Summers arrived in Sunnydale, she was your typical female teen-an obnoxious little monster who spent more time getting under Buffy's skin then actually helping out at all.
As Dawn matured in Sunnydale and was assimilated into Buffy's family, it was a formidable time. Actually, it was literally a formidable time because before she showed up, she didn't have any form at all. You see, Dawn is The Key, which is an epically powerful energy force that a Hellgod named Glory wants to exploit to open up a portal that will release hell on Earth. Fortunately, a group of monks transformed The Key to the flesh and then added her to Sunnydale's memories and past. The monks' plan was to have the Slayer protect The Key from evil.
It wasn't for several months until Buffy and the Scooby Gang found out Dawn's true identity. While fighting to save the life of one of the aforementioned monks, Buffy was told the truth and then passed on the information to her mother Joyce, who was unfortunately struggling with a brain tumor that she didn't end up beating. Like any average bratty teen, Dawn didn't much like finding out that she was a cosmic force in the universe. As a matter of fact, she ran away from home. With little family left to speak of, Buffy decided to treat Dawn and the memories that came with her as authentic.
Sadly, last spring, Glory kidnapped Dawn after learning that she was The Key. During a bloodletting ceremony that almost brought complete anarchy to Sunnydale and earth, Buffy gave her life to save her sister, The Key. As it read on Buffy Summers' gravestone: "Beloved Sister. Devoted Friend."