I would go to the police right away.
The longer you wait the more difficult it will be to prosecute
Now...is important to understand that the police have "discretion"...the police are not required to investigate the case...they can decline to. But it is certain they will not investigate if you do not ask them. So you start with asking them to investigate
Now...what you describe, it sounds like fraud to me.
If there is evidence that someone other than your father signed these checks, that sure sounds like fraud.
The police will have the ability to subpoena bank records...so they can can look into her bank accounts to see if the money wound up in her bank account. If so? That would sure seem to indicate fraud
The great thing about handing this to the police (assuming that they take it) is that the criminal court system has the power to order her to pay "restitution"...so they can order her to pay the money back to the estate. That saves you the time and effort of a civil suit.
You mention the FBI...unless there is evidence of "interstate theft
" it is unlikely the FBI would take this...they do not get involved unless there is a very large amount of money AND there is evidence of more than one state involved. But the local police can investigate...and if there is evidence she is the one who took the money, they can prosecute.
Bottom line: I would report this to the local police and request they investigate the theft.