I wouldnt worry about it too much, I come from a big family, 7 sisters 2 brothers. I am a twin, me and my twin fought horribly in our early teen years to a point we didnt talk for a long while, now were best friends. And I have noticed, that the siblings in my family that fought the most with each other are best friends now. All sisters, all siblings go through it. My mother once told me that girls fight like that to prove their maturity, and by giving them more responsiblility, and showing more respect they quickly out-grow it. Let them do more things seperately, I know that no matter what, I always eventually missed my sisters company and enjoyed our time together that much more. And since we were separated with our different interests we were too busy filling each other in to fight.