Honestly, just walk away.
If you really think this could endanger you and your family, just leave it alone. Your father-in-law's demons don't need to become yours. The knowledge of this weighed on him and scared him into inaction for decades, and now he's passing that burden on to you and your wife? It's kind of a dick move on his part to transfer that to his own daughter's family.
Sorry if that answer seems selfish or cowardly, but in case you get a bunch of responses urging you to play private investigator, I'd encourage a more reasonable approach. What matters more to you - the safety of you and your family, or alleviating the guilt of the already deceased?
Good luck to you either way.
I don't quite know what it is - the rounded corners, the simple design, knowing they're for kids - they just make me happy to look at. Good work!