. . . The most likely source of
Fanny in your relative's case would be a Latvian or Yiddish version of
Frances, or a Latvian or Yiddish name which resembles
Fanny or
Frances. I had a quick look and the only obvious condender was
Freyde (click hyperlink for details), however I have no idea whether this was ever popular in Latvia - information on Latvian names is elusive unless you speak Latvian!