Ciao Misopogon, come stai?
I'll give that one a shot. Let me list some words for you to consider - perhaps the particular term you are looking for might be among them...
I have taken them from the online English - Greek dictionary by S. C. Woodhouse at:
It is easy to use and a great resource indeed. Try it out
Here are the words for brother:
And of course
as whiteoctave said
as you yourself pointed out
Now, these terms may not all carry the same meaning. If you find the word you're looking for, let us know.