I seem to be the only person looking for Lubans out there, but I'll keep trying.
My great granfather was Solomon Luban and I have found the family of his brother (Henry Luban). I have a single record of a Berko Luban who seems to have been a brother (similar age, father's name same, etc). I now have also found a single photo of a David Luban taken in London probably sometime around 1903-1905, give or take. Presumably David Luban was a close relative of Solomon and Henry from Rezekne, Latvia (then "Russia"). But no one seems to remember any brother other than Solomon and Henry.
If anyone has information of a Berko (Boris??) Luban or a David Luban who would have been around 25-35 in the early 1900's.