He most likely isn't as all the Darkin are bound to weapons, and I don't think Tahm kench's hat fits that description.
I am guessing he uses the term "brother" more out of a sense of kinship rather than literally being kin. Tahm is a very old long lived demon who devours those around him. Aatrox is a Darkin, which are pretty similar to Demons, and would probably be considered the same by much of the world, who has been around for around as long as Tahm and is considered a demon of war.
It makes sense that they would know each other and be more likely to be on neutral to positive terms rather than being enemies.