in november harold henthorn was charged with murdering his second wife in criminal trial which will also hear that he killed his first wife .
both died in freak ` accidents ' to which he was sole witness and fbi spent months investigating second death before he was charged .
in both cases , henthorn was the sole beneficiary in a string of lucrative life insurance policies .
his first wife 's sister-in-law , grace rishell , said that henthorn took out a $ 400,000 life insurance on her .
henthorn reportedly forged her signature and made himself sole beneficiary .
rishell believes she could have been the next to die in ` freak accident '