A part of me can't help but wonder if the use of "Chief" by the Fraser's to refer to a female isn't just another instance of the abandonment of the feminine version of titles in English-speaking society. For example, many females who write books are now referred to as Authors instead of Authoress. And of course there are female Actors know instead of Actress.

Or it could be that it is the office that matters, such as the use of Lord Lyon even when that title was held by a female in the 60's because Lady Lyon would have referred to the wife of Lord Lyon and not to the office holder.