It's her usual thing. Just ignore her.If you're gonna say that, then you...I'm gonna be center stage at the next concert!A sea urchin, is it?!You, big sis Merlin?! You look like you're wearing a sea urchin on your head!Listen closely! She's actually saying "I'll beat youuu"!
"Butsu" (ブツ)means "to hit", "to beat"Maybe a starfish or something?I'll be in the center!Sea urchin!?I'll beat you!!Quit arguing.Yours looks like a shooting star!!Lyricaaa, our big sister's muttering to herself againnn.
"Butsubutsu" (ブツブツ)is onomatopoeia for mutteringWha...hurry, big sis Merlin! Play some wild music for her!Beat you.Beat you...What're you talking about? It was decided that I will be, wasn't it!?Very literal, aren't you, Merlin?