@AlexEliot The text on the last page says the exact opposite of your translation. "電気つけなよ" means "turn on the lights". The な in this case is used as a command, not as a prohibition. It's hard to tell these apart in text as in speech the tone of voice is used to differentiate them, although there are some cases where the word form gives it away(not this time though). Looking at the context it comes clear, however, which it is. First, な prohibition is generally very unpolite and using it with your family members usually either implies a tense atmosphere or an angry or overbearing tone. From the impression on the brother it's clear that that's not the case. Second, it's hard to tell but from the first panel of this chapter and from how the window casts a light on her in chapter 114 and other small clues, it's also implied that the room is dark. Third, it's not an iron rule but sentences ending with よ generally do not use な in the prohibitive meaning unless it's a (intimidating) warning, which it isn't in this case. In this case the よ ending turns the commanding な into a plea, which however does make sense.