We know of the two Salvatore's, but we didn't know about the third, Genevieve Cordelia Salvatore, they weren't really related, but they did consider each other as full siblings. Lily and Giuseppe had found her on the steps, she was only about a year old and they took her in. At the time, Stefan was 5 years old and Damon was 12. In 1864, Genevieve Cordelia Salvatore watched her two older brothers die. Before she could do anything, her father dragged her back to The Salvatore Manor, locking her into her room. She was only 14 at the time, she went 7 years without ever knowing that they had instead turned into vampires, She knew only that Giuseppe had died, she had been sent away before he died to live in New York City with a friend of his, Daphne Morgan, who was a witch, a powerful one at that. It was there that she had found out she was a witch, and spent those seven years learning and studying. She was a very powerful witch, in fact she had a very close connection to nature, more so than other witches. She eventually ran into Damon, You can imagine her surprise when she had found out Damon and Stefan were alive.
Within a month of running into Damon, She disappeared. Her apartment trashed and bloodied, as if she had fought against someone. Damon looked for her for many years, but eventually had lost hope she was still alive. Unknown to him, She laid sleeping in a coffin buried in an unmarked grave in Mystic falls, trapped in a sleeping curse. She stayed sleeping for years, trapped in the coffin alone. She slowly started absorbing the sleeping curse, her eyes changing from gray to purple in her resting state. That curse, slowly seeping into her, changed her, she became a form of immortal, she had no hunger for blood, no weakness to vervain or sun, She became simply immortal. She lost none of the abilities of humans, but was something far greater.