Sisters? Sister-Wives?

This cabinet card portrait caught my eye because a typical family portrait would feature the mother, father and baby. But this one is different. Which lady is the mother? A close look at their faces suggests that the two women are sisters, but it seems odd for a sister to be included in such a family photograph. Then again, it was during this era that elements of the Mormon Church were still actively practicing polygamy, so were both of these women this man's wives? It seems far-fetched in our modern era, but such things really were different in places in the 19th Century; recalling such differences is one of the adventures in researching old photographs.