The Collaborative International Dictionary of English v.0.48:

Coheiress \Co*heir"ess\, n.
   A female heir who inherits with other heiresses; a joint
   heiress.
   [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

CO-HEIRESS. A woman who inherits an estate in common with other women. A
joint heiress.