My brother is my best friend as he helps me in everything which even our parents and friends can't do. We cannot rely on our friends on everything but siblings are always reliable.They can also help us in our studies. Therefore, we can state our siblings as our best friends.
When I got married, my spouse became my best friend. That's how it works--when you get married, your spouse becomes everything to you until you have children. Marriage means usurping your sibling as your best friend and placing that role on your spouse who is your loyal lover but also your best friend in sickness and in health.
I have one sibling. I am female and he is male. There is a fifteen year gap between our ages and we are far from being best friends. It is not that we do not get along, but the age gap made things very different for us as I joined the crowd when he was 15. I don't know many teenagers that want to befriend a baby.