This is the sibling-relationship between untriggered evolved werewolf, Grayson Salvatore and siphoner, Stefanie Salvatore.
Grayson and Stefanie are siblings; with Grayson being the eldest and Stefanie the youngest.
Grayson met his younger sister Stefanie when she was born on December 24, 2027; also known as Christmas Eve, at Mystic Falls Hospital.