Marco is the biological son of Marcel Atreides and the step-brother of Jowy Atreides. Due to his blood relation to Marcel he was treated better than Jowy.
When Marcel disowned Jowy, for allegedly being a City-State spy, and cast him out of the Atreides household, Marco displayed a lack of affection for his step-brother. His last words to Jowy were to tell him he wasn't his big brother anymore.
Following Jowy's ascension to the throne of the Highland Kingdom, Marcel, along with Marco and Rosa, relocated to Harmonia, because Marcel feared retribution from Jowy.