Exactly the same age difference between me and my younger brother. Unfortunately if they don't get on then this situation will continue for more years to come. I don't think you can resolve a clash of personalities. Me and my brother would argue and fight a fair bit but we could get on also. My mum never intervened in our squabbles and when either of us sought her alliance she refused to favour either of us. This ensured that we resolved our issues on our own. If things where getting too out of hand she would make us sit on a chair each in the lounge until we could be civil to one another! This I remember I found boring as she would switch the TV off. When me and my brother were arging it would only take one of us to say at some point "we'd better stop or mum will come and make us sit". As we both had in common that we wanted to afford this punishment we learnt to compromise so matters didn't escalate. Didn't always work as we could both be very stubborn but in the main it did.
Looking back I realise my mum was 'timing out' for all of us as she would never stay in the same room as us but would come back in if she heard us getting off our chairs!
If the situation is so bad then you should encourage them to play separately. Only punish them if things between them get violent and instead of taking tangible things away then threaten them with not being allowed to do something they each want to do in the future. I recall if mum prevented me from doing something I was looking forward to, this really hit home for me and I knew she would carry out her threats!
I think the best advice though is not to start involving yourself in their arguments nor trying to interrogate who's at fault as you'll only get drawn in. Stay calm and merely say 'if you two can't get on then I suggest you each go and do something else' -take on the role of referee.
Don't get yourself in a tiz, I'm sure all siblings fight and you know what they say "you always hurt the ones you love the most".
If you ask yourself "should one of them die would the other not be upset?" and the answer is yes, then I suggest counselling!
Most of all don't let yourself feel a failure, you cannot make them get on!
I hope my experience is of use to you. Me and my brother get on fine and we still see each other regularly even though we didn't socialise together much in our teens and early twenties as we didn't share the same social circle.
Keep us posted. Things wouldn't right themselves overnight, it will take time and alot of patience and restraint from yourself.

Hi
My two boys are the same, they are 5 & 7 and cannot be left alone or all hell breaks loose. I feel in total despair, I know parenting will never be easy, but give me a break! My kids should be able to be unsupervised in a room together for short periods surely?! I mean I should be able to hang the washing up, or make a cup of tea shouldn't I? I sometimes feel like walking away and never coming back but I couldn't do that to them or my husband. I don't know how I'm going to get throught this until they are old enough to be self sufficient. I feel like I'm 3/4 of the way to a nervous breakdown already. I think I'm headed for the looney bin so I totally sypathise. Kids drain every bit of enthusiasm in life you may have once had. All that said I do love my kids... honest! I would have left a long time ago if I didn't!!

I have lots of problems....but one think I don't have is kids who fight a lot. My kids are 9 (boy) and 5. (girl)..

They have different interests and hence can spend time doing their own thing...also the little one accepts that her brother is older and lets him take the lead a bit. She is easygoing and shrugs things off quickly.

Maybe try and do things alone with the kids a bit more and let them develop different interests...

also make sure dad gets involved more...

and a reward system might help.

I fought a lot with my siblings (4 of us ..very close in age)...but now we are all very close....so keep up hope..

Good luck