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)

Moby Thesaurus II by Grady Ward, 1.0:

64 Moby Thesaurus words for "sisterhood":
   affiliation, agnation, alliance, ancestry, blood,
   blood relationship, boon companionship, brotherhood, brotherliness,
   brothership, chumship, club, cognation, colleagueship,
   common ancestry, common descent, community of interest,
   companionship, comradeship, confraternity, confrerie, connection,
   consanguinity, consortship, country club, cousinhood, cousinship,
   enation, esprit de corps, fatherhood, fellowship, feminism,
   filiation, fraternal order, fraternalism, fraternity, freemasonry,
   guild, kindred, kinship, maternity, matrilineage, matriliny,
   matrisib, matrocliny, motherhood, order, paternity, patrilineage,
   patriliny, patrisib, patrocliny, propinquity, relation,
   relationship, rights of women, secret society, sibship,
   sisterliness, sistership, society, sodality, sorority,
   ties of blood