Sibiling Rivalry

I know there is a standard amount of conflict that arises between siblings...however, with my son Ryan and daughter Clara, that level is spiraling out of control. I believe they hate eachother. And I'm at a loss to know how to fix it.

Everything Clara does bothers Ryan. Her typical 2 year old behaviors send him into a sensory overload that usually results in anger and a meltdown. She doesn't understand how to share or that sometimes objects belong to other people. So when she 'steals' something of Ryan's, he flips out. He hits her. He kicks her. He slaps her. He cries and screams and pulls his hair and hits his head with a closed fist. He stomps and slamps doors. These behaviors also come after she enters his bedroom (she doesn't have to do more than just walk through the doorway) or if she does something she's not supposed to but that doesn't involve him. For example, the other day, I asked her to hand me something and she threw it on the floor instead. Before I could even move to respond to her behavior, Ryan had grabbed her arm and yelled "CLARA! NO!" He feels like it's HIS job to discipline her, to reprimand her. He doesn't understand that what WE do is not okay for HIM to do.

On the other hand, she is not an innocent bystander. She knows how to get him riled up and she does it on purpose. She takes his toys, she screams at him, she pokes and prods him, she scratches him...she only does these things to him. She waits for the perfect opportunity and she strikes. It's really bothersome to me. I feel like she's bullying him. And she's hardly 2 years old!

The stress of this situation is really starting to wear on our family. I don't know what to do. Neither of them really understand. Ryan says things like "I hate Clara" or "Clara is too loud" or "I don't like Clara"...and about Nicholas (who can be equally annoying) he says "The baby is SO cute!" or "Look at Nicholas!" It breaks my heart.