In Buffy the Vampire Slayer in the season where Dawn, Buffy's sister, was new and the hidden key, Buffy finds out through a ritual that Dawn is something different and not her real sister. Her mother finds out because she has her mental episodes and Buffy fills in the gaps.
But how does Giles know? In the middle of the season (5), they have a secret discussion between Buffy, Giles and mother, while Dawn overhears their discussion.