17mo DS keeps lashing out at 7yo DD, help!

[jennieflower]

I posted this in Behaviour / Development yesterday but got no reply. Can anyone here offer any advice?

My lovely little boy is turning into a monster! Over the last few weeks he's taken to scratching, slapping, headbutting and hairpulling, and most of it is aimed at my poor DD.

It usually happens when we're all cuddled up on the sofa together, and he tends to go from being affectionate to being really brutal. So far I've made a point of making a fuss of DD whilst ignoring him when he hurts DD, then asking him to give his sister a cuddle. He seems to really enjoy the making up part of being told off, so when he's finished his cuddle he'll headbut her on the nose really hard, then giggle whilst I tell him off and go back for another cuddle.

Poor DD, I'm not sure how to handle this, What's the general concensus when it comes to disciplining a child so young? Any ideas?