Seriously, on what planet did Grams ever have the "witch" talk with Phoebe, Piper and Prue? We are told that when the girls received their powers, they had no idea what this entailed. Ominously, the girls were "told" that now that they had their powers, demons would come after them.
Second, we know from Season 3's Pre-witched that Grams kept the attic locked from the girls. Apparently, none of the sisters knew that Grams was a witch, let alone that their mother had been one. From Season 2's - admittedly horrible - How to Make a Quilt out of Americans, we are told that none of the sisters was aware that Grams's coven met at the manor.