What kind of family demands members sign NDAs?
And if what was written does violate the NDA then it must be true because the only violation would be to reveal actual facts. By demanding that this is covered by NDA only adds further credence to the claims she's making. You can't claim it is all lies while at the same time claiming it is an NDA violation with any shred of logic.
@DB, the article Chosen posted gave the answer to your question, but I'll add a few details. When Fred Trump died, Donald Sr.'s brother and Mary (brother's daughter) decided to contest the will. Part of however the lawsuit was settled - not to Mary's liking, evidently - was that
all concerned signed an NDA. IOW, Donald Sr. is one of those bound by the NDA (though how Mary's violation of the NDA affects Donald Sr.'s obligations under the NDA, I won't guess).
NDA's are not particularly uncommon. I'm probably bound by several - NDAs signed after changing jobs, whether voluntarily or by layoff. They are fairly common in settlements of civil lawsuits. If Mary doesn't like being bound by an NDA, maybe she should have thought of that predictable aspect of however
the lawsuit she and her father filed would be settled before filing the lawsuit.