You first need some factual information. I would contact ATF directly and ask an agent what would be required in this case. There are provisions in the law for exactly this event, so reading the regulations on the ATF site would be helpful. The executor of the estate should have already done this.
If the estate will not comply with the applicable regulations then there is the option to file a lawsuit or claim against the estate if it goes into probate.
But the lesser of two evils may be just to simply ask around and see if a friendly local FFL would act as an intermediary for the estate for $25. The administrator of the estate should have done this already.
Local shops step in to handle estate liquidations all the time if none of the heirs are up for it.
