I have been trying to find information on this brother for a number of years - without success. He was the only one of my grandfather's siblings who remained in the USSR after the Revolution (my grandfather had moved to Finland by then) and is known to have lived in Leningrad since sometime in the early 1930s. He apparently moved to Tallinn sometime in the late 1950s or early-mid 1960s. My grandfather got a permission to visit his brother once in Tallinn sometime in the mid- 1960s, but after that all contact with this branch of the family was lost.
I would much appreciate any help or suggestions on how to get additional information on these persons, or records of family members maybe still living in Tallinn etc.