Jealousy

Daughter has started playing a little rough with little G. She's even taken to hitting him if either me or hubby tells her off for doing something naughty and he's in her local vicinity. If he's not then it could be me, or the walls, or even the furniture that gets a punch.
I've noticed that because he's started to roll over by himself she tries to do it for him even more now, using his mat instead of actually turning him so there's more of a chance that he might get hurt.
Also once he's on his front she thinks she can ride him like a pony...
I mean I know hes 17lb and 8oz (got him weighed on Wednesday) but blimey he's only 6 months old. Oh and when he's in the pushchair and she tries to climb in and sit on his legs oh my life!
I know its a bit of jealousy coming through and I know that most of the time its a healthy thing that she's not afraid to show us she's jealous but after speaking to my mum...
She told me about a family she knew and they had 2 kids about our kids age and the youngest was in the cot with one of his arms poking through the bars and the other child managed to break the arm and the parents had them both taken from them because the social workers thought it was the parents that did it....
I don't want that to happen with our kids.