Children who tattle tale...  

I have 3 children. My oldest is my daughter who is 4 and my youngest daughter is 3. My stepson is 5. My 4 year old is a devil. She likes to pick on her brother and sister. My 3 year old just comes and tells me with no emotion. My stepson just sits and cries about it. Punishing my daughter just does not work anymore. I tell my stepson and my 3 year old to fight back. I dont know what else to do. I dont want my children to be wimps. My stepson always just takes crap. His little 16 month old cousin beats him up like pulling his hair, pushing him and other things. He wont defend himself. How does any other parents deal with this sort of thing?