The Collaborative International Dictionary of English v.0.48:

Suster \Sus"ter\, Sustre \Sus"tre\, n.; pl. Susters,
   Sustres, or Sustren.
   Sister. [Obs.] --Chaucer.
   [1913 Webster]

         There are seven sustren, that serve truth ever. --Piers
                                                  Plowman.
   [1913 Webster]