Is the questioner's mother sinful for not informing him that his wife was frequenting sorcerers, and is his divorce of his wife due to her ill-treatment and her sorcery against him considered justified?

If your mother is certain that your wife is in contact with sorcerers, or strongly suspects it, and conceals it from you, she is mistaken. This is because informing you is not incitement to divorce, but rather advice. Backbiting and tale-telling are permissible for the purpose of giving advice and averting evil. However, if her suspicions are mere delusions, then she is not obliged to inform you. In any case, do not distress your mother, nor confront her with what she dislikes, even if she falls short, for her right is immense. And for your information, the cure for magic is through ruqyah (Islamic exorcism), and it is not permissible to seek help from sorcerers and charlatans.