I'm trying to find a book that I read probably in the late 90's early 2000's.
It's an urban fantasy. The premise is that there is a young girl who finds out she comes from a family of witches.
I only remember some details:
There is a special house that all of the women in the family go to.
The girl has a grandmother with dementia who wanders off all the time, but when she goes into the house her dementia goes away.
The girl has an older sister called Helena or Helen, which gets shortened to Hell. (I don't think she's magic)
I think it might be set in or near Wales, as they eat Laverbread.
The girl is given a necklace which is a gold disk, maybe with a small gem set into it, that's special in some way.