Who has a right to a deceased persons' financial records?

As there is a beneficiery, insurance will pay out only to that person. The executor will have no rights to the payment but can report The death. The executor cannot touch it but will file all finances to the court. Same goes for a joint account.
You need to file with probate before the wife gets there first that you are family and willing to be executor

I agree, contact the Probate in Dads County. He can get a short certificate that gives him the ability to get to Dads finances. He may have to hire a lawyer, or at least go to Legal Aid, to have a letter written that she is to turn over all Dads financial records. She will have to show proof of marriage to be able to claim the right to Dads records. If Dad owned a house, had a sizeable estate, ur fiance can become an Administrator of the estate if no Will. Dying without a Will means this women is entitled to nothing. The State Dad resides in determines who inherits. Dads children will come first.

There is nothing filed in probate court

Contact probate court in the county where father resided.