If he has a will (and I'd be shocked if he doesn't) then fair is not what it is about.
If he doesn't have a will and he is worth tens of millions, the federal government will be sure to rake him over the coals.
I think your mother needs to approach your uncle and grandfather about the subject and just ask them to update her on your grandfather's estate planning situation. Ultimately it is not really her business. It just depends on the family dynamics how you broach the subject in a way that doesn't set off someone. If your uncle is trustworthy, and if he and your grandfather have any sense, they will have had some estate planning to work around inheritance tax laws.