XML schema two-level key/keyref constraints

Given the following xml:

<root>
  <families>
     <family familyname="Flintstone">
         <member name="Fred"/>
         <member name="Wilma"/>
     </family>
     <family familyname="Rubble">
         <member name="Barney"/>
         <member name="Betty"/>
         <member name="Bamm-Bamm"/>
     </family>
  </parents>
  <employees>
     <employee name="Fred" familyname="Flintstone"/>
  <employees>
</root>

Is it possible to build a key/keyref constraint in the xsd to verify the validity of empployee?

The following does not work for key (the keyref is straigt forward):

<xs:key name="Individuals">
    <xs:selector xpath="./families/family/member"/>
    <xs:field xpath="@name"/>
    <xs:field xpath="../@familyname"/>
</xs:key>

Is there a valid alternative?