Not gonna be perfect, but something along the lines of:
"Eeeh? You, are a ghost user too!"
"O-Onee-san also uses ghosts"
"If it's okay, c-can we have a battle?"
"I-if you win, onee-san will do… whatever you want~"
And in the background it says "I want to be friends" over and over.

Surely you can't be suggesting…