You might have discovered the "smoking pen".
I suggest you take the documents (and any other proof) you possess and have them reviewed by a lawyer.
With a million US dollars at stake, dropping a couple hundred bucks is worth it to avoid being fleeced, bamboozled, scammed, and conned by a crafty relative.
The lawyer will then advise you about next steps.
If things are as you say, you want to stop the bleeding, attempt to recover any leftover purloined funds, stop further thefts, and make sure the guilty parties are dealt with according to the civil and criminal laws of your state.
Until you know more, just remain mum, play nice, and don't tip your hand that you smelled smoke, located the smoking gun, initiated a search for the loot, and cooperating with authorities to make sure all criminals receive their comeuppance.
This is NOT a DIY effort, which is why I suggested licensed legal help in your county.