What is the ruling on a wife calling her husband's mother "my mother," "my aunt" (maternal), or "my aunt" (paternal), and likewise a husband calling his wife's mother by these terms, given your conflicting previous fatwas and a fatwa from Ibn Uthaymeen that does not permit this?

It is permissible for the husband to address his wife's mother as "my mother," "my aunt" (maternal), or "my aunt" (paternal), and similarly for the wife with her husband's mother; because the default rule regarding customs is permissibility. The intention behind such addressing is to show kindness, honor, and to spread affection and amity. Matrimonial alliance is among the blessings Allah has bestowed upon His servants. The Prophet (peace and blessings be upon him) commanded good treatment towards the people of Egypt due to ties of kinship and affinity.