Professor (Sergio) and Berlin (Andres) were not originally supposed to be brothers. That’s mean in real life they are not brothers.
Berlin and Sergio in Money Heist
Professor (Sergio) and Andres (Berlin) have created their own backstory for their characters, in which they are half-brother, and Berlin is the older brother from his father’s first marriage. The reason why they can speak and understand some Russian is that Berlin took his younger brother to Russia for medical treatment when he was young.