The Ghost of RubangB on 23 February 2009
According to the Super Mario Adventures and Mario vs. Wario comics, Wario is a childhood rival of Mario's.
According to Nintendo Power, Wario is a cousin of Mario and Luigi.
According to the Colonel from MGS, when telling Snake about Wario in Brawl, Wario was a rival of Mario's.
But I don't think any of those are canon. If Yoshi's Island DS is canon, then Mario, Luigi, and Wario were all born at the same time, along with Donkey Kong and Peach.
