Really like the style and design of the artwork, very nice. And for the record, there was an interview somewhere with Charles Martinet (don't know where or when, just remember reading it) where he said the Mario brothers were cousins to Wario and Waluigi, so it's pretty likely that the two are brothers, or at the very least closely related. And if anybody doesn't know, Charles Martinet is the voice of the 4 characters in question.
It's not really sure if they are brothers but most fans like to think they are. I see them as big brother (wario) and li'l brother (waluigi). I think it'd be cool if they were thicker then theives and were brothers that really loved one naother. Like Wario would totally give Waluigi a hardtime but totally protect his younger brother when it counted. The two can just jack people left and right too.