At first I thought the gag was that Reimu pulled the same prank, but thinking about it Rinnosuke probably asked Reimu to help him prank Marisa, which is even funnier
Please make me your bride!Reimu and I are already—I'm sorry, Marisa...QuietlyNssh-ssh-ssh♡Kourin...no, Rinnosuke-san!...not!♪ April Fool's, da ze!