It's a trap. Soon as the Marisas pull the feed close enough, the Reimus will jump on and start eating. Naturally, that will anger the hard working Marisas...who open their mouths to protest such uneasness. Whoosh, Reimus are Mr. Birds.
My guess is that the adult Reimu is also the mother of the Reimu who is getting beat up. She dumped the kid to be with the adult Marisa, and the two scumbags are the result. So they're shit kicking their own elder half-sister.