Whether you might be the youngest, middle, or oldest of your siblings there is a bond there that is hard to break. For me, I am the baby of one full sister and two half siblings that I admire. But my sister is my number one. God made us to be sisters knowing that we would always need each other, and also to annoy our parents.
With a sibling as a best friend, family vacations and parties are never dull (especially if your family is already crazy). It is a constant person to always turn to talk to, to laugh with, and to always have someone to agree with you. When you have to share a bed while away, it's not a big deal because you just lay there laughing at stupid stuff. Or if you're like me and my sister we choose to share a bed because cuddling is what we do.
If you're lucky enough you can attend the same university as them. That gives an instant person go lean on when college tries to kill you. It's a person you don't have to start all over with, you just continue being there for each other. If you're the youngest you have the chance to be a legacy in Greek life, so you become double sisters or brothers. You have a person that saves your ass whenever with whoever. Thank you for saving me at my first glow party and nights that do not end well.
No matter what, I will have the most trustworthy, kindhearted, and caring person to always have in my life as my best friend. They're the person to call at anytime about anything. The person who knows how to deal with the parents. Siblings are there to annoy the hell out of each other, take things, get to aggressively mad at, but most importantly to be a best friend.