While it would be extremely hard to rebuttal a religious statement of a christian grandmother [i know.. i have one] and have her listen to and take seriously anything you say that disagrees with her.. This inst truth.
the bible itself is full of magic, performed by and for Christ and his followers. in first Corinthians chapters 12-14, Paul talks about "spiritual gifts" that are given by god which include: healing, miraculous powers, prophecy,distinguishing spirits, speaking different languages, and the ability to interpret different languages. [1corinthians 12:7-11]
these gifts and practices [which are the same ones that wiccans/"white witches" posses and/or practice] are all given from god.
now, your grandmother may say these people Paul was referring to in the new testament were "christians"/followers of Christ. they used their gifts to worship god. and she may make an argument that because wiccans [not all] dont believe in god and/or worship him that their power is "satanic". But this is taking a very closed minded and blind eye view [i mean absolutely no disrespect to your grandmother at all]
we cannot view the world [especially god, and spirituality] as black and white. we cannot simply say "if it is not of god then it is of the devil". this just simply isnt true. just because certain people, [born with or attained through study and practice] posses the same powers that Christians claim to have come from god, yet do not use them for the good of god.. that does not automatically mean they use them for evil.
and furthermore we can not just assume because these gifts are not being used in god's name that they are consequently being powered by satan.
im not saying satan has no funding in the supernatural gifts that some people possess.. just that it is unfair, and prejudice to label an entire group of people as "satanic" because they are not followers or worshipers of the christian god.
especially a group of people such as the wiccans, who actually practice a loving and respectful form of magic. [most]
just my personal thoughts :)
[your grandmother would love my memaw]
peace and love