WIBTA if I continue calling my MIL after my SIL asked me to call her Mrs. [last name] 3 years into my marriage

My SIL and I/my husband had a disagreement/misunderstanding about something unrelated, and in repairing that, we each asked each other what we could do better. I was expecting the response to be related to the disagreement we had, but instead she told me I should call my MIL (her mother) by Mrs. [last name]. This