B.C.
I have a 13 & 10 year old & a 6 year old but all mine are girls. My 13 & 10 are the worst about fighting and it has been known to get physical with them. I tend to seperate them when it gets really bad and ground them when it gets physical - no phone, no going ANYWHERE & life gets very boring for them for atleast 2 weeks, no exceptions ( be aware this also becomes a pain in the butt for me) - however it does go the trick for awhile. I tend to put it off more on my 13 year old because she is older and I expect her to know better - the 10 year old is hanking her chain to get a reaction and she fell for it.
The one thing I do stress to them is that while I understand their feelings and know how aggravating siblings can be (I am the oldest of 3 and we very close - now)and no matter what happens inside the house they are NEVER to allow anyone else to pick on their sisters. Allowing this brings swift and harsh punishment. And on some level this works because I have heard my oldest telling her friends to back off my 10 year old and I have seen her stick up for both of them in public.
My hope is that they will outgrow it and eventually become close with each other as I did with my brother and sister.