Am I sinful if I do not call my husband's mother "Mama" after having sworn to do so and then said it habitually, and how should I deal with this, and is an expiation for breaking an oath (kaffarat yameen) incumbent upon me?

The fundamental principle for someone who has sworn an oath and then broken it is that an expiation for the oath becomes obligatory. Jurists have differed regarding the obligation of expiation when the oath is broken due to forgetfulness or ignorance, and similarly, if the reason for the oath ceases to exist, then expiation is not obligatory. Love should prevail among Muslims, especially among relatives. There is no harm in addressing non-parents with "O my father" or "O my mother" out of respect, and it is better not to abandon this practice as it fosters affection. She should be treated with kindness and patience when wronged, especially if she acknowledges her mistake and initiates reconciliation, as this is an act of righteousness towards the husband and may strengthen affection.