I spent the last five turns stealing back any stars that Wario and Waluigi managed to obtain by depositing more coins to the windmills they owned (they only had two stars). Then on the last turn, I used a Slow Shroom Orb to land on a Duel space and I beat them at the minigame and stole all of their coins. They were left with nothing at the end.
By the way, I wasn't Team Peach and Birdo. I was actually, um...Team Daisy. The reason why Peach and Birdo have so many stars is because I gave them an 8-star handicap at the start of the game. Gotta make sure that the least popular characters win!