Depending on the names that are on the letters and who is mentioned in the letters, you can try going to one of the local genealogist in your area. You would be surprised what they can find. My wife does genealogy. A year and a half ago she started working on my son-in-laws family and was going to give it to them as a Christmas present, this past Christmas. She finally gave a rough copy to his family a few weeks ago that contained over 4,000 names. Every name was verified and included birth and death certificates, marriage licenses, photos, obituaries, grave stone photos draft notices and anything else she could find to verify the accuracy. So I'm sure that a genealogist can trace those letters to who wrote them and who received them and their relationship to your mother.