I think I said, since they have letters and correspondence, they could use DNA to screen candidates. I agree, if they had to start cold DNA by itself wouldn’t be much good.
Unless they are parents, or multiple sibilings, not cousins, aunts, uncles or further generations, there is very little possibility of any match. And that is dependent on people from 80 years ago being placed in a DNA database.
Kinship Analysis
http://massfatality.dna.gov/Chapter12/KinshipAnalysis/