She is super strong and apparently doesn’t have charm that works on people. Or she’d have it easier in life than she has. She might either be half or simply the variety witches is broader than the one we know for sure seen.
I see a few possibilities:
Her mom is a witch and her dad is a human so she's a half-witch with half as many powers. This would imply that full witches are born some other way (since men can't be witches). Maybe witches can have kids with each other or maybe they summon a baby out a magic ritual or they just sort of spontaneously appear or something (I would guess the first one).
Her mom is a witch and her dad is a human but witch-ness is a dominant trait so she is a witch and half-witches don't exist.
Both her parents were human but witch-ness is a recessive trait so both parents were surprised by her being a witch. Or she just happened to have the witch gene as a mutation. This explanation doesn't explain why we never see her mom, whereas in the other two scenarios we can assume she left because her husband found out she's a witch.
In scenario 2 or 3 we're still left with the question of why she can't do the things Sadako can. A likely explanation would be that being a witch works kind of like in Harry Potter, where it's an inherited trait but you still need to train to be able to use your powers properly. It could also be that witches are born with different powers available to them. In that case, we could say that Sadako specializes in manipulating people's brains (mind controlling, memory wiping, perception alteration, invisibility) while Hanako specializes in physical strength (telekinesis?). This wouldn't explain some of the other things Sadako has done, like flying, telekinesis, and whatever was killing the people who entered her old house (Usually in a setting where everyone gets a unique power set like that they're quite specialized and don't overlap, but it could also be that every witch just has a random selection of spells she can cast or that some powers are universal to all witches but others aren't.)
Personally, I lean towards the idea that Sadako is a full witch who just hasn't studied how to be one. Maybe learning to do magic and appreciate her powers will help her to accept herself as a witch?
Also, big thank you to the translators! I'm already extremely invested.