They are less than a year apart, and generally get along really well. DS can be extremely protective and loving towards her and they share toys, food etc nicely.
Recently though I've seen if they are left alone when he is tired or frustrated he gets seriously nasty towards her. One occasion last week I went to the loo and heard a huge commotion, to find he'd trapped her inside the toybox, and was sitting on the lid laughing as she hysterically screamed.
He also tries to take food and drinks away from her, even when he has been given exactly the same and she just lets him. It's heartbreaking as she idolises him and he can be so cruel.
Tonight was something else though and he is currently in bed crying, DD is asleep in our bed with DP.
They share a room, DS in a toddler bed and DD in a cot. Sometimes he climbs into her cot and they sleep together, which he did this evening and it's always been fine so I turned out the light and left them.
I then heard proper screaming and dashed into their room. He was kneeling on her chest with her hand in his mouth, he had fucking blood around his mouth. I'm ashamed to say it but I pulled him off her and threw him to the floor. Her entire hand was covered in bite marks, and really deep ones, bleeding everywhere and her index finger was blue and swollen. How could he have done so much damage in less than a minute? We consoled her, cleaned her up and put tcp and plasters on but her finger kept swelling and I saw she couldn't move it. DP took her to A&E and she needed to have skin glue on one wound and an x ray showed a small break to her finger. They have taped it to the next finger, and also wrapped a bandage over her whole hand as she's too young to know not to mess with it. I am furious, with DS but mainly with myself. We sat him down and showed him she is hurt and he said sorry, but he doesn't even know the true meaning.
How am I supposed to stop this sort of thing happening? What am I doing wrong?