I think they were cousins, at best, but Decimus was a cousin of Caesar’s, and both of them participated in the conspiracy and the assassination. He was also designated as Caesar’s heir if something happened to Octavian (it sez here).
As things shook out in the months after the assassination, Octavian and Antony were to all appearances opponents. Then Decimus’ army (the murderers will still in charge in the senate; they should have known better than to trust the rest of the senate) which he had been given to chase down Antony and take over his territory, deserted. They deserted to Octavian. Decimus fled, trying to reach Cassius, the better-known Brutus, and that lot, over in Greece, but was hunted down by Gauls in the employ of Antony. Buh-bye.
That particular episode was shown but altered in the HBO series “Rome” — young Octavian was chasing and beating Antony, but the fictional version has Lepidus (the eventual third member of the Second Triumvirate) as the commander that had his army leave for Antony. An event *like* that took place, but the Decimus character was never introduced, probably to simplify the story. The historical Lepidus seems to have had a good head on his shoulders, and yet wasn’t shy about lowering the boom when he had the advantage.
http://ancienthistory.about.com/od/caesarpeople/f/111708Brutus.htm
http://www.britannica.com/EBchecked/topic/82460/Decimus-Junius-Brutus-Albinus
http://en.wikipedia.org/wiki/Marcus_Vipsanius_Agrippa
http://en.wikipedia.org/wiki/Marcus_Aemilius_Lepidus_(triumvir)
Yet Suetonius names Brutus as Decimus Brutus. Is this a different Brutus, a cousin? Is that what you mean?
http://www.historyofwar.org/articles/people_brutus_decimus.html
http://www.britannica.com/EBchecked/topic/82460/Decimus-Junius-Brutus-Albinus
http://en.wikipedia.org/wiki/Second_Triumvirate
images:
http://www.google.com/search?q=decimus+brutus&sa=X&oi=image_result_group&tbm=isch
http://www.google.com/search?q=lepidus&sa=X&oi=image_result_group&tbm=isch