The testator cannot bequeath the property to which he does not have marketable title.
If your uncle has allotted more property than his entitlement, then administrator of the will can either accept to execute only that share to which the testator was entitled or to face the court proceedings for a decision to be taken by court in a suit filed by the co-lessees.
The solution lies in an amicable settlement or through a court of law.
The other shareholders can issue a notice to this effect to the administrator seeking to refrain himself from proceedings with unlawful activities as described in the notice or else to face the music of law in the court of law.