Zach is a desendent of Stefan and Damon but were not exactly sure how he is related to them as I dont think Stefan and Damon had another sibling. Maybe he was related to them through an aunt atau uncle of Stefan and Damon making them his great, great, great...uncles. This leaves the possibility that there are lebih human Salvatores out there.
In season one a man named zack lived in the Salvatore house. He was the son of stefan and Damon’s half brother. Zack had a daughter named Sarah, Stefan secretly gave her away in an adoption. Damon knew nothing about Sarah later on in the tunjuk she died because sybil stabbed her.