Complex sentence: Peasley learned of Cackletta's plot and warned the Princess beforehand, so they disguised Birdo as Peach, who had her voice stolen instead of the real Peach, explaining the Beanstar's reaction to "Peach's" voice.
Split into simpler sentences: Peasley learned of Cackletta's plot and warned the Princess beforehand. They disguised Birdo as Peach, who had her voice stolen instead of the real Peach, explaining the Beanstar's reaction to "Peach's" voice.