Where would you be without friends? The people to pick you up when you need lifting? We come from homes far from perfect, so you end up almost parent and sibling to your friends - your own chosen family. There's nothing like a really loyal, dependable, good friend. Nothing.
I grew up on a farm in Oregon, an adopted child, with one sibling, and parents the age of all my peers' grandparents. We lived in isolation from the people around us, and it was always a struggle to cope with as a child. The heart can really expire under those conditions. I always felt like I was looking at the world from the outside.
I believe in soulmates, yes, but I believe you also have to work at love. I happen to believe your soulmate doesn't have to be your partner - your soulmate could be your best friend, your sibling, it doesn't have to be the person you marry.
If you're an older sibling and you have a younger sibling who needs mentoring or is afraid of the dark, you develop nurturing and empathic skills that you wouldn't otherwise have.
There's a sort of sibling moratorium when you're establishing yourself as an adult. So much of your energy has to be focused on other things like work and kids. But when people become more settled, siblings tend to regroup because now you're building a new extended family.