The Collaborative International Dictionary of English v.0.48:

Sisterhood \Sis"ter*hood\, n. [Sister + hood.]
   1. The state or relation of being a sister; the office or
      duty of a sister.
      [1913 Webster]

            She . . . abhorr'd
            Her proper blood, and left to do the part
            Of sisterhood, to do that of a wife.  --Daniel.
      [1913 Webster]

   2. A society of sisters; a society of women united in one
      faith or order; sisters, collectively. "A sisterhood of
      holy nuns." --Shak.
      [1913 Webster]

            The fair young flowers . . . a beauteous sisterhood.
                                                  --Bryant.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sisterhood
    n 1: the kinship relation between a female offspring and the
         siblings [syn: sisterhood, sistership]
    2: an association or society of women who are linked together by
       a common religion or trade or interest [syn: sisterhood,
       sistership]
    3: a religious society of women who live together as sisters
       (especially an order of nuns)