No contract shall be let to or made with any person in which any officer of the department of transportation is interested, directly or indirectly, or with whom any officer of the department is knowingly related, either by blood or marriage within the fourth degree, computing by the civil law. Nor shall any stockholder or other person in any way interested in any firm or corporation be related within the fourth degree to any member of the department. Neither shall any engineer in the employ of the department have any interest in any contract or be knowingly related within the fourth degree to any person interested in the contract. Any contract where the interest or relationship exists shall be void.