I hope this message finds you well, present circumstances excluded. There is no law which would preclude relatives from being co-witnesses to an event or testifying on one another's behalf. A law such as that would violate the equal protection clause of the Constitution, among other things. As long as all of the witnesses are willing to sign affidavits, or sworn statements, as to what they saw or what happened, then Progressive has not legal leg to stand on in this instance.
Progressive is banking on the fact that they believe you will not take them to court if they deny the claim. You can take them to court and use the same sworn statements, police reports and photographs to get payment for the windshield and even court costs and related damages.
Do not relent in this endeavor. If they continue to deny the claim, tell them you want the relatives excuse in writing. In any event, file this in small claims court in the county in which it occurred against both Progressive and the trucking company.
In summary, there is no law like that which Progressive is attempting to cite against your case as long as they are willing to be sworn witnesses to the facts surrounding the event.
Let me know if you have any other questions or comments.
Best wishes going forward!