She could ask them to test without revealing that it is concerning an NPE. But, actually, why does she want them to test at all? She already has the results of *four* of the grandchildren.
I would not tell them. It's (possibly, only possibly) giving her peace, while at the same time shattering the peace of two people, possibly/probaby more (their spouses, their children, your uncle if he is still living, your aunt ditto, his living sibling, and so on). And once done, knowledge like that can never be taken back or forgotten. The peace of the many should outweigh the peace of one.