He just forgot that he is talking about his wife's parents. His wife who has accepted his family so beautiful inspite of all the tantrums whether it is mamiji's taanas or Ishwari insecurities. She had been caloing his mother maa even when they were in relationship.
There were other ways of relieving Ishwari's insecurities.
By committing that he will not call Sonakshi's mother maa, Dev has really proved himself again as a spineless dumbhead son