younger sigling displaying symptoms of bad behaviour. Older brother has adha

My nephew who is 14 has Adha, and conduct disorder and possibly ASD. My nephews younger sibling who is 4yrs old, is displaying the same behavioural patterns as my nephew. However, in my opinion the 4yr old seems to be far worse in his behaviour. My nephew at that age was not as bad in his behaviour. I have taken a lot of notice of the 4yr olds behaviour pattern. How i mean by bad behaviour, is the 4yr old has a younger sibiling 3yrs old. What is concerning is how the 4yr old constantly hits the 3yr old. Really hurting him. What really alarms me the 4yr old has tried to pull the 3yr olds private parts  off, The 4yr old will take everthing the 3yr old has eg. food, drinks,toys. The 4yr old with put items of the 3yrs old into the rubbish bin. The poor 3yr old has no say. He is hit kicked pushed,nipped. Always has black eyes. lumps on his head, bust lips. ect. The 4yr old also bullies, hits, kicks, other children in school. Teachers are very concerned. And i agree. Something is wrong with the 4yr old. His behaviour is a cause for concern. Need some helpful advice