sister and brother 

It is two persons of the opposite sex who are always there for each other. They can not even be related but trust each other anyways as if they were 'brother and sister' These 2 persons can never lose faith in each other because they will need each other. In one way or another they will always love each other no matter what, and no one can separate them.

I love have a sister and brother relationship.

sister and brother by Grid_Dzyr November 18, 2013

Brothers and Sisters 

Referring to the testicles, and the sperm therein, of a man who has one or more children.

While sitting on the floor changing my daughter's diaper, she's started flailing her legs around and kicked me in the 'brothers and sisters'.

Brothers and Sisters by vankinney September 1, 2009