What is the ruling regarding a 12-year-old younger brother supplicating against his siblings with evil? Will he be held accountable for it, and how can he expiate for that?

A brother's prayer against his siblings in a moment of anger is not answered, because the supplicant does not truly intend the supplication nor is his heart attached to it. Nevertheless, efforts should be made to refine this brother and educate him about the seriousness of anger and how to treat it. If he is an adult and prays against his siblings without justification, he must seek forgiveness and supplicate for their well-being. However, if he is not an adult, nothing is incumbent upon him, and the path for him is advice and discipline.