Could the fluctuation in the child's academic excellence and his aggressive behavior toward his sister be due to envy, and what is the advice for that?

A child at this age (adolescence) needs special care, and the reason for his declining academic performance should be investigated. This decline might be due to conceit resulting from excessive praise, or neglecting studies and behavioral correction, or spending a long time in front of the television, computer, and internet, or associating with bad friends.

Parents should draw closer to the child and make him feel important, and set a specific study schedule for him without depriving him of play and recreation. The effect of the evil eye should not be ruled out, as the Messenger of Allah, peace and blessings be upon him, said: "Indeed, the evil eye can put a man in the grave and a camel in a cooking pot." And he, peace and blessings be upon him, used to seek refuge from it by saying: "I seek refuge in the perfect words of Allah from every devil and poisonous pest, and from every evil eye." If the cause is known, the cure is easy.