Selek, son of Sasak and T'Pel (actually Mr. Spock in disguise)
Selek, son of T'Pel and Sasak, was a distant cousin of Sarek. In 2237, while journeying to the family shrine to honor the Vulcan gods, Selek visits Sarek's home in Shi'Kahr, where Sarek offers for him to stay with them, to break up his journey. While visiting, Selek's young cousin Spock, prematurely begins his kahs-wan across Vulcan's Forge. Selek follows Spock and is able to save him from death when he and his pet sehlat, I-Chaya, were attacked in the foothills of the L-langon Mountains by a wild le-matya. (TAS: Yesteryear")
Background
In reality, Selek was the identity assumed by Spock when he traveled to his past, through the Guardian of Forever, in order to save himself from otherwise being killed by the le-matya. The names "Selek", "T'Pel" and "Sasak" could possibly be true distant cousins of Sarek's family.
The character of Selek would later be echoed by a similar scenario in TNG: "Firstborn" where Alexander Rozhenko traveled back in time to visit himself, posing as a family friend known as K'mtar.