19mo - hitting and pulling hair

My 19mo boy is going through a difficult stage of hitting my eldest daughter (6yrs) or pulling her hair hard for absolutely no reason. He isn't bothered if I tell him off so this trick just doesn't work. What can I do? I feel so sorry for her as she has to take the brunt of it and I always have to justify him just being young and not really understanding his actions. My boy is so so different to how my girl was, he throws all of his toys and will unintentionally wack you over the head or in the face if you're not careful. He also smacks the TV and doesn't stop until I drag him away. It's a large screen TV and is within easy reach as it's quite low down. Again, he isn't phased if we tell him off and often he just finds it funny. This is really stressing me out, he is otherwise such a lovely little boy and loves his sister so much. I just really hope he isn't as bad as this at nursery. Is anyone else going through this stage?