What will you do? Why?
Your brother studying in XII has developed the stress.
There are various reasons because of which my brother may have developed stress. Most possible reason is, that he is in class XII and is worried about his board exams and his future. However, my first step would be to sit down with him and understand the cause of his stress. After understanding the cause, I would work with him to chalk out a strategy to solve his problem.
I would talk to him in a manner that promotes him to think positively and his mind gets diverted from negative thoughts. Another strategy would be to suggest him to indulge in his favourite activities.