Amara was a human, but the Petrova family sorta were witches. They were travellors, there is a diffrence. Katherine was originally a witch, because of her father being a Travellor.
"The Travellers are a mysterious millenia-old group of witches who have presumably been transmitting the knowledge of their spells from one generation to another since their emergence around the time of the Roman Empire, in modern Europe. Silas and Qetsiyahwere said to be the most powerful members during their time. However, Silas became immortal, rendering him unable to practice magic, and Qetsiyah was later killed by the group for reasons that have yet to be explained."
All travelers are witches, but not all witches are travelers.