I really like the game theory to that

The baby versions of most mario characters, including Wario, were canonically stolen by Kamek in yoshi Island 2. At the end of the game, all children are returned to their parents expect for Wario, who steals all the gold of the island and flees with a raft

So the theory would be that Wario was waluigis brother but went missing as a child, and the two are just not realizing it.