What's a Sister Anyway?

My parents tried to conceive for something like 10 years by the time I was adopted.  By the time they would have had money to adopt more, I suspect that it would have been at a time in their lives where they weren't in the baby-phase or expanding the family phase.  Though, they did keep trying to conceive after I was adopted; my mother had a miscarriage when I was four.  I was raised an only child but always wondered if I had brothers or sisters somewhere out there.  I was delighted to find out upon reunion that I have three brothers; two are younger than me, one is older than me.  Then the thought crossed my mind....I have no idea what being a sister really means.

I ran through the relationships that are kind of close:

Cousins: a cousin, to me, was always someone who lived far away.  Who I was related to but didn't know anything about.  We shared the same grandparents.  Our parents were brothers and sisters.  No, my friends who had brothers and sisters seemed closer to their siblings than how I had been with my cousins.

Friends: I have close friends I would do just about anything for.  Our families are close, but as close as siblings?  I'm not sure.  My friends seem closer to their siblings than they are to me.  No, being siblings seems more meaningful on some level than being friends.

Well, I'm a mother but being an older sister isn't really like being a mother.  They have a mother.  We share a mother.  I'm protective of them but I'm not responsible for their care nor am I in charge.

Figuring out how to be a sister is hard stuff.