Esther is the "Original" witch - meaning the witch from the Original family.
The Original family believed they were the first vampires, because they did not know about Silas. Kol later learned about Silas, but only that he was "immortal" - they only learned he was a vampire much later. Also it is unclear if Silas has sired any other vampire. If he has not, the "Original Family" remains the "Original Family" because all vampires in existence today were sired by them.