Having a sister is pretty much the best thing ever — she's a built-in best friend. My little sister Maddy was born when I was 3 years old, so she's basically been around since I can remember. We laugh together, we bicker with each other, and she knows all my secrets. Even when she drives me insane, I know she will always be there for me through the good times and the bad. Maddy is a best friend for life. Here are 20 reasons why any sister is a BFF for eternity.