She punched a punching game machine and it broke down. The establishment made them pay for it. The place was run by the yakuza, no doubt. It wasn't a good idea to give them his address and contact information.
I thought that this was just some tsun/dere Freudian split, but damn if that dead eyed child sitting in the void don't wreck that theory. Her exuberance suddenly makes a lot more depressing sense.
There is this manwha in webtoon with similar premise but instead of here where the person inside you leave the body they instead transform you into a monster that follows the person’s desire.
In the sister’s case she’d be a monster that will chase onii-chan