I am not an OK attorney so I could be wrong but based on a quick search OK doesn't currently have a state inheritance tax or state estate tax. In any event, if the state did levy such a tax and if the people you say are entitled to the account are so entitled, and the assets are taxable (IF!!!) then the funds would still be taxed even before they are technically moved because the rights in the account vested upon death. Again that's a lot of "ifs"
This is not legal advice nor intended to create an attorney-client relationship. The information provided here is informational in nature only. This attorney may not be licensed in the jurisdiction which you have a question about so the answer could be only general in nature.
In a nutshell, you and your brothers can not owe a tax that does not exist. And if we were still back a few years when it did exist, the amount you reference is well under the threshold from back then.