I still don't see any evidence that it can be taken as a general term for another wife of one's husband. Certainly in the case where the relations between the wives are not entirely peaceful and where there is the additional complication of the one being unable to bear children I can see the term being used, but not in the general case. Certainly not if the women work well together. My understanding is that many in that situation today find it quite pleasant to have sister-wives.