I have to agree with the previous responses that you're making an awful lot of assumptions and guesses that may not ring true. This may be due in large part to you not having much information for yourself, which is perfectly understandable.
Probably the best answer to your ultimate question is to suggest that you wait until your grandmother passes. I'm assuming that you've already been to the County Clerk and read your grandfather's Will that was admitted to probate. If you haven't done that, do it. You won't be assuming and guessing anymore. I'm also assuming that by your reading of the document, you will have some standing as a beneficiary when your grandmother dies. If you aren't sure, take the document to an attorney to read it for you.
If you're correct that your grandmother has a lifetime right to all of the estate (which wouldn't be odd) but no right to direct how any of the remaining estate passes (which would be only a little odd), then you are entitled to see the documents that grant you any of your rights. You would be entitled to see her Will, or the Trust, or whatever document confers upon you the right to inherit.
This answer does not constitute legal advice. I am admitted to practice law in the State of Texas only, and make no attempt to opine on matters of law that are not relevant to Texas. This answer is based on general principles of law that may or may not relate to your specific situation, and is for promotional purposes only. You should never rely on this answer alone and nothing in these communications creates an attorney-client relationship.
One's probate estate consists of assets that were in the person's name at the time of his or her death. It does not include assets that pass outside probate, such as jointly-held property or property for which there was a designated beneficiary. Your posting did not provide any information about how the approximately $5 million held by your grandfather was held -- while you assume that his will provided for his assets to be held in a marital trust until his wife's death, you don't actually know. You similarly don't know any of the assets was held.
Likewise, you say nothing about the provisions in his will. We don't know whether your mother was a named beneficiary of the will, or whether the will provided that a beneficiary must survive the testator by a fixed period of time in order to inherit, or whether assets bequeathed to a person (i.e., your mother) would abate in the event of the person's death or whether such assets would pass to the issue (descendants) of that person.
What you can do is consult with a local attorney. In doing so, you should bring a copy of your grandfather's will and provide all of the information that you know concerning the assets of the estate.
Good luck to you.
Michael S. Haber is a New York attorney. As such, his responses to posted inquiries, such as the one above, are limited to his understanding of law in the jurisdiction in which he practices and not to any other jurisdiction. In addition, no response to any posted inquiry should be deemed to constitute legal advice, nor to constitute the existence of an attorney/client or other contractual or fiduciary relationship, inasmuch as rendering legal advice involves the ability of the attorney to ask appropriate questions of the person seeking such advice and to thus gather appropriate information. In addition, an attorney/client relationship is formed only by specific agreement. The purpose of this answer is to provide the questioner with general information, not to outline specific legal rights and remedies.
I agree with the prior poster regarding the fact that there are just too many unknowns here to answer your question. The real key is what Mr. Haber mentions about "per stirpes" distribution. The will and/or trust has to be read to determine if your mother's share passes to her children (you) or not if she fails to survive your grandfather. This is typical in many wills, but not required or absolute.
This is not legal advice nor intended to create an attorney-client relationship.
You may have a case. A lawyer would need to review the Will. Just because your mother passed doesn’t mean your generation was to be excluded. It is not uncommon for those with POA to say there are no assets anymore. You need to talk to attorney and give them your supporting documentation if any.