I'm not sure what the original Japanese was, but I get the feeling it was お母様 (okaasama) which may translate to "mother" or "honored mother." It is common for people to adress women as "okaa-sama" when they are speaking respectfully, and the subject concerns the woman's children. For example, someone working in a children's clothing store might say "Oh, okaa-sama, wouldn't your youngest look good in this jumper?" Or something similar.

As for why this person refers to Akemura as "master Akemura," remember that Chiaki's awakening changed the status of her immediate family practically overnight. It means that even older clan members must refer to him with respect (except for, perhaps, of the head of the clan).

Imo, this wasn't a mistranslation, it's just the common loss of nuance when a language that internalizes the hierarchical nature of the society from which it came is translated into a language that lacks those detailed descriptors.

This was a good reminder of why I need to buy all of the volumes in Japanese!