How often do you have family functions? How many times has she brought this up? I’d let it go. If she finished high school in the early 60’s, I assume your relative is now over 70? Soon your kids will be out of the age group where NM means anything to them. If NMS is the best thing your relative has on her resume after all these decades, well, that is sad. I feel sorry for her. Let it go. Let her have her (now pathetic) moment of pride. You/your kids don’t need to try to discredit this old lady or do a lot of research to try to prove her scores weren’t really so hot. For you to do that would be even sadder. (I’m not sure if the information is available/comparable anyway.)
(We have 5 NMSs in our immediate family–nothing against it. We’ve collected some great scholarships. But I don’t brag about it to the relatives.)