In Super Mario Odyssey, Bowser kidnaps both Princess Peach and Cappy’s sister Tiara. While Bowser kidnapping Peach is to be expected, I don’t understand why he kidnapped Cappy’s sister.
Tiara doesn’t display any special capturing powers like Cappy, nor does Bowser seem to have any grudge against the Cap Kingdom. The only thing kidnapping Tiara does is give Cappy a reason to team up with Mario.
Is it ever explained why Bowser kidnapped Cappy’s sister Tiara?
The storyline of the game is “Bowser steals the fanciest things for his wedding to Peach”; Tiara is presumably a high-value wedding prize for his bride to be. In the next three levels (after Cascade), he steals a dress, bouquet, and ring for Peach was well. The tiara was just the first step in his quest to deck her out in the best duds available.
Source: wedding items section