Okay so this is a Japanese story ( we don't bow to each other in apology like Mary did to the wife) and the translators get confused because there actually is no title Mother - in - Law in Japan. See, in Japan, a woman marries INTO her husband's family, literally becomes her husband's mother's daughter. O-KA is the Japanese word for MOTHER. A Japanese woman would call her mother in law Ka-san, because you always follow KA with an honorific. SAN is formal, CHAN is informal. If she's got a good relationship with her birth mother she would call her Ka-chan, fondly. I agree that the In-law thing shouldn't be that hard to remember, but I notice it always does seem to cause confusion amongst translators. (The same goes for FIL, he is now her father, equal with her birth father, and because you always refer to a grown male formally, she would refer to both as O-to-san or To-san.)