What is the ruling on a wife cutting ties with her wicked, non-Muslim mother who practices sorcery, and is the husband sinful for preventing his children from seeing her?

If it is confirmed with certainty that the wife's mother practices magic (sihr), then she must be advised and urged repeatedly to repent. If she does not repent, there is no harm in severing ties with her if there is a benefit in doing so, such as deterring her. There is also no harm in preventing children from visiting her if you fear harm to them from the magic. It should be noted that a mother's right to dutifulness (birr) is not forfeited by wickedness or disbelief, and connection is not limited to visitation. There are other degrees of connection that remove the sin of severance, such as inquiring about her and making phone calls.