The sisters symbol is therefore depicted as two women surrounded by a circle.

A circle around other Native American symbols signifies family ties, closeness & protection. The circle has no break and holds that which cannot be broken. The circle is used as a basis for many symbols including the cycle of life from birth to death to rebirth and is suitable as a symbol for sisters. For additional information refer to the Family Symbol.

The Meaning of the Sisters Symbol
The symbol of the sisters depict two women who are positioned side-by-side which represents unity and equality. The line connecting the women symbols indicates that they are sisters who are bond together and represents a shared journey through life. The Symbol for Brothers  are represented by a similar sign. 

Should one woman symbol be positioned above the other this may be interpreted as a Symbol for Mother with a daughter and son or as a symbol of a grandmother.

The Sisters Symbol - Marriage, Co-Wives and Sisters
The Sisters symbol within the circle has additional significance due to the role of the co-wife in some Native American cultures. The word polygyny describes a marriage in which two or more women share the same husband. The term  Sororal polygyny describes a marriage in which the co-wives are sisters. This was a practice in at least 40 Native American cultures during the 1800's. A marriage in which the co-wives were sisters was often preferred because sisters are thought to be more mutually supportive and less argumentative than non-siblings. In the tribes that followed this practise the eldest sister in a family married first and that as they came of age her younger sister or sisters would join her as co-wives. Among the Lakota Sioux, two men who had pledged life long friendship to each other might express this relationship by marrying sisters and exchanging wives on certain occasions. Another custom relating to marriage and sisters was that when the wife of a man died, he would often marry one of her sisters. The practise of Sororal polygyny was described by a Navajo tribal chief as follows:

"A man would marry a woman, then work hard for his family. If she had a sister who was not married,
and if the man proved to be caring, a good provider, and a good husband, he would be gifted
with his wife's sister, marrying her as well."