Blood is thicker than water
Our loyalty to our family — that is, to our blood relations — is strong no matter how we may feel about them.
Examples from the Web for blood is thicker than water
I wonder if I can work the blood-is-thicker-than-water touch on him for a fiver afterwards?The Boy with Wings|Berta Ruck
Idioms and Phrases with blood is thicker than water
Family ties are closer than other relationships. For example, Nancy will drop everything to help her sister; blood is thicker than water. Alluding to the fact that water evaporates without leaving a mark whereas blood leaves a stain, this proverb was first recorded about 1412.