Sisters

1. a female offspring having both parents in common with another offspring; female sibling.
2. Also called half sister. a female offspring having only one parent in common with another offspring.
3. stepsister.
4. a female friend or protector regarded as a sister.

The first four definitions of sister from dictionary.com

Also, –adjective 12. being or considered a sister; related by or as if by sisterhood: sister ships.

Those you consider sister, whether by blood, marriage, accident or simple act of life...

Real sisters, like real family, aren't born, they grow to be by word and deed - they are more than simply a product of blood they are a product of life. And that means, that though I was born a third generation only child, I consider myself blessed by the sisters in my life.