From wikipedia, “…the ability to shapeshift into a wolf or an anthropomorphic wolf-like creature, either purposely or after being placed under a curse and/or lycanthropic affliction via a bite or scratch from a werewolf, or some other means.” Granny related the hereditary history of their family – the family has the affliction of lycanthrope.
We don’t know how the affliction started – so it could still be a family curse. [Unless I totally missed that bit of information during the episode.]