Marco is the son of Marcel Atreides and is Jowy's step-brother. He is Marcel's own child and not a child from Marcel's wife, Rosa's, previous marriage like Jowy. Marco received better treatment during his youth than Jowy and, like his father, he often expressed his dislike for Jowy.
When Marcel disowned Jowy and cast him out of the Atreides household, Marco felt no angst toward Jowy's inevitable departure. After Jowy was refused to be allowed to see his mother, Marco made his good-byes and stressed the point that he was no longer Jowy's brother.
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, whom he had treated poorly during his youth. It is presumed that Jowy has not spoken to his mother, Marcel, or Marco since his disownment.