Wario and Waluigi aren't twins, but maybe brothers.
It's kinda odd. I have Mario Party 5 and when you pair up Wario and Waluigi, they are known as the "Wicket Bros." instead of the "Wario Bros." If they were related, I guess the latter name would've been chosen as their name for their team. Though, one thing you have to notice: Waluigi only appears in certain spinoffs with Wario. When it comes to the games like WarioWare, Waluigi is nowhere to be seen.