Does a prenuptial agreement matter for a marriage dissolved by death?

State of Michigan, if that helps.

My grandpa, after 50 years of marriage and two daughters with his first wife, decided to marry again in his elder years to a nice widow from his church. They are both of independent means, and signed a prenuptial agreement to the effect that if the marriage is dissolved, their (adult) children, and not each other, retain the property of the estate. Further, my grandpa stated in his will that he made no provision for any property for his current spouse, as she has her own means and property.

However, when he died, the house he bought with his own money (not hers) was in both of their names, and contrary to both the pre-nup, his will, and his oft-heard verbal assertions that the house will go to my aunt, it is reverting to his second wife (and because she’s suffering with Alzheimer’s, the house will either pass to her power-of-attorney-holding son, or perhaps be confiscated to pay for her assisted care, before Medicare kicks in to pay.)

I realize that this is a legal question and requires a lawyer to answer, but I’m also wondering if anyone here might know offhand if prenuptial agreements are used only in the case of divorce, or if the death of one of the spouses would also be the occasion where the pre-nup matters. As my aunt isn’t fighting for the house, it’s mostly a matter of intellectual curiosity; hence no need to consult an actual lawyer. I’ve seen online that a pre-nup can be used to “ensure that your estate is distributed according to your wishes upon your death, and is protected against additional claims by your spouse,” (source) so I’m also wondering if perhaps the lawyer who told my aunt that she doesn’t get the house is, himself, fulla crap.

As always, thanks for reading. :)