My kids fight all the time! Before we got our newest 2, DS 8 & DS 4 were at it all the time. Now all 4 have their moments. They are 8, 6, 4, & 4. I remember fighting with my sisters a lot growing up, but we are wonderfully close.
My kids fight like we did, snippy and ugly, but won't actually hurt each other. I think they will have the same type of realtionship that I had with my sisters, we could pick on each other, but watch out if someone outside of the family tried too!
I think the only way to avoid it is to only have one kid!