Siblings have a unique relationship. It's more than the relationship of best friends—although I know siblings who are truly best friends. I've seen this with my mom and her sister and my own daughters. This can also be true of brothers and some brother-sister relationships.
The relationship of siblings is unique because they share similar DNA. This can be a good and bad thing and leads to what I call the love-hate element of sibling relationships. They can be each other's protectors or advocates then arch-enemies within minutes of each role.
I've seen this over and over again in my own family and with other siblings.