DD1 is currently 8 (9 in June) and DD2 is 6. They seem to be constantly at each others throats. It seems to come more from DD1's side although DD2 is far from an angel.
I know that some bickering and fighting is to be expected and it's part of their development in looking out for themselves in the big wide world and bladey bladey blah! but how much should you intervene.
My two are currently in another room but I can hear them talking together. DD1 has just told DD2 that she hates her and thinks she is horrible. It breaks my heart to hear them talking like this to each other.