6 year old fighting with little brother

[Flugelpip]

I have a feeling I'm going to get a lot of people saying 'me too' but it's the fourth week of the school holidays and my two DSs (almost 6 and 3.5) are fighting a lot. A lot. The almost 6 year old is big and strong for his age, and has a temper. The 3.5 year old winds him up, pinches and kicks him, and generally provokes extreme violence - he's part of the problem but he comes off worse because he's smaller.

The older one was a very gentle baby and toddler but since he started school he has been more and more physical, and has got in trouble for hurting his classmates (in play). If anything, he's worse now than he was at the end of term and I'm worried about how he'll be when he goes back to school in September. He just pushed his little brother so hard he fell over and cut his arm and leg. He wasn't one bit bothered about being told off until I said he couldn't look at the injuries he'd caused, at which he went into screaming fits of rage. I've sent him to his room but I don't really have any punishments that seem to work. I've reasoned with him. I've explained to them both that they can't behave that way. Neither of them cares, at all. And I'm not very good at staying cross - they both know that all they have to do is make me laugh, and if one's in trouble the other one clowns around until I'm back in a good mood. It's not bothering either of them except when they actually get injured, but I'm exhausted from telling them how to behave and worried about how they'll be in future.

They are capable of playing nicely. They get on quite well. But they are both very competitive and increasingly aggressive, though only with each other - and I'm out of ideas.