Blood is Thicker than Water

No, really. It is.

The viscosity of blood is about 3-4 cP at 37  °C .  The viscosity of water is 0.890 cP at about 25 °C.

So?

Well, this little tidbit of science was first turned to metaphor by Heinrich der Glîchezære, author of Reinhart Fuchs (circa 1180).  His intent was to illustrate his belief that familial ties were stronger than the bonds of friendship ever could be.  Obviously the masses believed there was some truth to this, as his Blut ist dicker als Wasser became a common proverb still used today.

In 1920, Aldous Huxley put a slightly different spin on this phrase in "The Ninth Philosopher's Song:"
Blood, as all men know, than water's thicker / But water's wider, thank the Lord, than blood.

That is obviously open to many different interpretations, mine being: Screw viscosity.

Jim Butcher, I believe, said it best:

“When everything goes to hell, the people who stand by you without flinching -- they are your family. ”