There's six years between middle sister and myself and as kids we hated each other and were really horrible. Mum spent most of her time refereeing. However although disciplined for bad behaviour towards the other, the message was very firmly that whether we liked each other or not we were sisters and it was my duty as the eldest to protect younger members of the family, (something that has carried on to this day). So while it was alright for me to be mean to Blondie, it was very much another story if I caught someone else doing the same thing.
I wouldn't say our relationship ever really improved much as children/teenagers but we learnt to rub along together and as we got older and found more in common we obviously became closer. I used to feel bad when she wasn't allowed to do things because of her age and as I go older would quite like acting the 'big sister' and buying her a sneaky archers if we were out or saying that myself and my friends would go out with my sister if mum would let her go out to see her friends in the evening. I suspect mum approved of this sort of ganging up on her of her daughters because it did mean that the relationship between Blondie and I grew stronger for it. It was even me that went with her to her college open days and parent evenings initially.
We're now very close even though she lives on the other side of the world so you can obviously grow out of sibbling rivalry. When baby sister came along a few years after Blondie, those two were very close and used to gang up on me if I was being mean so it's swings and roundabouts.
That said, the one time mum witnessed me slap Blondie around the face she turned around and did exactly the same to me while citing that I was twice the size of her and she was twice the size of me! It was a very short, sharp lesson and while I resented it at the time, the message was received loud and clear.