Thanks for your responses, guys. Sorry I couldn't reply sooner.
I like the suggestions, timetosmile. Getting them together rather than trying to separate them is a good one. I'll give it a go. We did home movies too recently. DS was fascinated (and a bit embarrassed!) at how much attention he got when he was an only child. Bless.
Andro - I think things have got worse, yes, but he did always to this to some extent. I think that because we're together a lot things are exacerbated. He's not the most articulate of people (another reason for his frustration with his very articulate little sister!); he mostly says 'she's annoying' or something similar. I'll try to encourage him to be more explicit. He did say yesterday that he does it for attention because he thinks she gets more than him.
I feel your pain! Don't know that I have any wonderful advice though, but do bear in mind that this is a really stressful time for him if you've just moved (school as well as house). Sometimes giving them a joint task for a cash incentive works wonders, such as 'if you clean the lounge and dining room including hoovering and dusting and tidying up' as a team with no squabbling I'll give you £2.00 each' Or get out the old photos and home movies and all have a laugh at how cute they were together aged 2 and 5..we did that the other night and 12yr DS and 9yr DD got all misty eyed and reminiscent (ahhh!)
Another tricky 10/11 DS. Besides lots of attitude to me, he's constantly having a go at his younger sister (8). He's always resented her, though they are also sometimes great playmates, but now he's always having a go at her, saying snide things, trying to wind her up. She's pretty tough and fights back (and so gets into trouble too - result for him of course) but it gets her down being constantly criticised and undermined. We've always used sanctions, set boundaries etc and I always talk things through with him but it makes no difference.
He likes school and behaves well. We recently moved so we're a bit short of social life so thrown together more than I'd like. Basically, it's all very tense with lots of shouting from all of us. I'm feeling overwhelmed and not sure how to deal with it. Any suggestions?