Source : Webster's Revised Unabridged Dictionary (1913)

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

         There are seven sustren, that serve truth ever. --Piers
                                                  Plowman.