coheiress

from 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]
from Bouvier's Law Dictionary, Revised 6th Ed (1856)
CO-HEIRESS. A woman who inherits an estate in common with other women. A 
joint heiress.
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