Can an executor deny siblings to look at the financial books?

The executor must file an account that lists all the assets of the decedent both real and personal property. That means all bank and investment accounts, vehicles, furnishings, art, jewelry, etc. That inventory is public and you can request the probate file and examine it. If you think the executor has not done a thorough listing all the assets then complain to the court. The executor will later be required to file an accounting to the court detailing how all those assets were distributed.