It's a shitty situation to be in. But i've seen people do the PI, legal, forensic accountant and it can take years depending on the situation. It is also expensive as most of the time there isn't much they find.
Granted the executor or executrix should have a legal obligation to act in the best interest of the estate. In addition, they should be able to provide all paper work and should be able to back every up with appropriate documentation too.
Money can break family apart. Seems like might be potentially a larger sum of cash....
Best of luck....