Does my uncle have the right to physically punish his nieces?

My uncle is babysitting us while my parents are away. He's becoming more aggressive, shouting at me and even at my grandma (not his mother). I'm afraid he'll resort to violence as he has in the past. Can he? When we texted my dad about it he replied to 'let him cool off'. He adores his brother and would let him get away with murder. My mother prefers not to get involved. We are aged 17, 16 and 5. What course of action should i take?