I can only imagine how it would be having a sister. I was raised as an only child so I mostly related to older folks at home when I was a kid. Now from my daughters I see that the sister relationship is one of both simplicity and complexity. Simplicity in that who else would be better to play with than my own sister, complexity in that I don't always want to play with my own sister. Intensity in arguing over wanting the same thing at the same time, intensity also in sharing and having so much fun as sisters together at the same time. Difficulty in figuring out how much room to take, difficulty in figuring out how much room to give.
However, despite all of this the one thing I know for sure about sister relationships is that they are special-only in a way that sisters can understand.