As mentioned, witchcraft isn't hereditary. My mother was a barmaid, and so was her mother. This doesn't mean that I am a barmaid by virtue of the two previous generations. I don't work in a pub, hence I'm not a barmaid. If you don't practice witchcraft, you're not a witch.
There is some lore within some branches of witchcraft in regards to a magical bloodline (red thread, nephilim, etc), but it's really just an attempt to establish oneself as somehow "better" than other witches. It's an ego boost, or sometimes just a demand to be recognised as legitimate. Granny stories are a really popular result of this.