Monster Kid refers to a "sister" once or twice in dialog, notably at 1:03 of this cutscene:
In Japanese, there are separate words for "older sister" and "younger sister," so I assume the Japanese localization had to pick one. Unfortunately, I do not speak Japanese. Which did they use?
Note that I am specifically looking for an answer based on the official Japanese localization. I am aware that the linked YouTube video contains Chinese(!) characters, but those characters are not relevant to this question because they are not official.