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-literally.  You see, Dawn is the Key, which is an epically powerful energy force that the Hellgod Glory wanted to exploit to open up a portal that broke down the barriers between the dimensions. Fortunately, a group of monks transformed the Key into flesh and then added her to Sunnydale's memories and past. The monks' plan was to have the Slayer protect the Key from evil.

Buffy discovered the truth about Dawn during a spell she cast to see if Glory was making her mother ill.  She talked to Giles but witheld this information from everyone for another month.  While fighting to save the life of one of the aforementioned monks, Buffy was told the truth.  She told her mother, Joyce, when in a moment of 'clarity', linked to her brain tumor, she saw Dawn for what she truely is--an empty shell.