Easter egg: On the 2nd disc go to storyboards. Go to the 2nd page and select rhino fight under Deleted Sequences. Press up once to select the rhino and press enter. This will get you to a section called What Happened to the Rhino.
Easter egg: On the 2nd disc select trailers and TV spots. When you are in this press left once, this will highlight the bird on his chestplate. Press enter and you will get a funny Chicken Run trailer.
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