Oh. Dawn Summers was Buffy’s sister and turned up in the fifth series of BtVS. The wiki entry sums it up better than I could:
Dawn was introduced at the start of the fifth season as part of a large in-story retcon: Characters accepted Dawn’s presence as if she had always been there, and as if Buffy always had a sister, with only the audience aware that this was not the case. As the series went on, the significance of Dawn’s arrival is revealed to the series’s other characters, and they come to understand that she has not always been Buffy’s sister, or indeed a sentient being; Dawn had originally been the mystical “key” to unlocking dimensions and was made into Buffy’s sister so the Slayer would protect her. Dawn is, however, a real girl, Buffy’s biological sister, and has real memories of her fictional childhood.
