Will the wife be considered sinful if she reduces her visits and calls to her husband's mother – while maintaining a minimum level thereof – due to the harmful behaviors and discrimination between grandchildren that emanate from her, especially since her husband agrees to this?

There is no blame upon you for reducing your visits to your husband's mother if it is proven that harmful actions originate from her. However, it is better to endure her harm to increase your reward. It is obligatory to raise children to be dutiful to their grandmother, for honoring grandparents is like honoring parents. The grandmother is not obligated to treat her grandchildren equally. It is advised to gently and kindly advise the grandmother regarding any actions she commits that are not permissible by Islamic law.