All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
We don’t know whether or not that white powder is magic; it may be so and it might not be so. If we presume that it is magic, then it is not permissible for your mother or any other person to accuse your wife of having done it. In principle a Muslim is innocent [until proven guilty]; Allaah Says (what means): {O you who have believed, avoid much [negative] assumption. Indeed, some assumption is sin}[Quran 49:12] Also, you are not permitted to believe whoever claims that your wife has done magic if he/she has no evidence to support his/her claim, whether it is your mother or that man who claims that he treats magic with the Quran; for more benefit, please refer to Fataawa 100199 and 90787.
In fact, if that man is pious, he should not dare to make such an accusation in principle, let alone informing your family about it, because this act leads to affliction and to the spoiling of relationships within the family. We fear that that man is one of the magicians and sorcerers as they most often do that.
As regards treating magic when it happens, or protecting oneself from it before it happens, then this is by mentioning Allaah, reciting the Quran, performing Ruqyah and so forth; for more benefit in this regard, please refer to Fataawa 84498, 82918 and 4754.
Therefore, we advise you to be wise in solving this problem; if there is no evidence for accusing your wife, then you should inform her that you believe in what she says, and ask her to help you in solving the problem by not causing any problems with your mother. Then admonish your mother in a soft and gentle manner and inform her that it is not permissible to suspect a Muslim without evidence.
Allaah Knows best.