My little sister is my best friend. It has not always been this way, but I value her friendship more than any other human being (other than my husband).
As children, our father pitted us against each other often sparking physical fights between us because he thought it was normal to hate your siblings. Then after our mother passed away, we lived separately because her mental health went into decline and she was in and out of homes, kicked out of our adopted families' home and moved from relative to relative until she finally got married. My little sister struggled with Bi-Polar disorder and we fought often, mostly due to the fact she felt I was not sad enough over our mother's death and felt I didn't love her enough.
When I met my husband he helped me cut her off completely because of her caustic effect on my emotions and well being. About 2 years ago though, she woke up, she lost a ton of weight, started feeling better about herself, and we made amends. I was cautious at first because we had made amends and broke ties many, many times over the last 15 years. It turned out I didn't need to be cautious though, and last December on the 14th anniversary of our mother's death, she called me telling me she was pregnant! We have had so much fun the last 7 months preparing for the baby and shopping for baby things and planning her baby shower. I finally feel like I am getting to be a big sister and that my little sister needs and looks up to me, and we talk every day!