Jealous siblings

I have a newborn and his brother is two years apart. My elder child will find any opportunity to beat his younger brother whether the adults are around or not around. I have explained to him that the newborn is his brother and must care of him but maybe he is too young to understand. He will grab his younger brother's clothes, throw something at him and violent towards him. When will he understand and not to treat his brother that way. Is too dangerous to leave them alone even for a second.