"I'll never let a simple DNA tell me who's my brother and who's not." (taken from www.desmotivaciones.es)

Maybe we don't have the same parents, neither the same family. But the simple fact I can share my problems with you makes us sisters/brothers. The fact that I can share what I like with you makes us closer. A brother/sister is not only the person that has the same parents as you, but the person that shares his/her life experience with you, and allows you to share your life experience him/her.