Hermano is the Spanish word for "brother" - a fact that neither Michael nor G.O.B. knew when they were tracking down the elusive man they believed was named Hermano. Turns out, Marta was interested in G.O.B.'s hermano - Michael. ("Marta Complex")
Buster clues G.O.B. in on the definition and they believe it is the man who plays Marta's brother Tío on El Amor Prohibido. Michael eventually confesses the truth to G.O.B., and the two fist fight over her in front of the courthouse. ("Beef Consommé")
G.O.B. evidently learned what "hermano" means and calls Michael "hermano" in "Best Man for the Gob".
- Buster states that Tío is Marta's character's brother on El Amor Prohibido. In Spanish, Tío means "uncle". This could be a joke by the Arrested Development producers (confusing "Hermano" for a name just to have "Tío" be a name) or a mistake on Buster's part.