Maximus (Russell Crowe) is a general in the roman army loved by Marcus Aurelius, the emperor. Marcus wants Maximus to become emperor after he dies as he thinks that his son Commodus is unfit to rule. When he tells Commodus this, Commodus murders him. When Maximus refuses to believe that Marcus died of natural causes, Commodus orders that Maximus and his family should be executed. However, Maximus escapes and returns home - too late to save his family. He collapses and is sold as a slave where his prowess as a gladiator gets noticed and eventually he fights his way through Africa to Rome, where he meets the emperor face to face. The emperor spares his life, then challenges Maximus to a fight in the arena. Maximus accepts and the two engage in a bloody battle. Maximus kills the emperor, reinstates the Senate as the governors of Rome and dies to be with his family in the afterlife.
Chosen answer: No, they are full brother and sister. They both had the same mother and father. Incest was not exactly normal at that time, but it stretches back within the imperial families as far as Caligula and possibly earlier. There is no evidence to suggest that the real Commodus was attracted to his sister, it was probably just included in the film in order to make the character seem more disturbed, and also as another reason why he would be Maximus' enemy (Maximus and Lucilla were, after all, once lovers).
The Doctor