i can understand where law would cme into t really. If the stuff is burnt, it's burnt, and she can't unburn it. ok, so it might not have been the right thing to do from your families point of view, and it certainly seems pointless, but what can law do about that? its not like marriage and birth certificates are intrinsically valuable, so it's not like they have lost anything of worth (in fact they havent actually lost anything as they never had them in the first place.)
as to you being told you will inherit 1/3 of the estate - is this what the will says?