Blood is Thicker than Water means: People who are related have stronger obligations to each other than to people outside the family.
Example of use: “When my best friend and my brother got in a fight I had to help my brother; blood is thicker than water.”
Interestingly, the origin of the idiom “blood is thicker than water” is believed to be a Scottish novel. The saying is attributed to the Scottish poet and novelist Sir Walter Scott, and is seen in his 1815 novel, Guy Mannering. Various forms of the idiom have been in use for hundreds of years.