“Blood is thicker than water” is a German proverb (originally: Blut ist dicker als Wasser.), which is also common in English speaking countries. It generally means that the bonds of family and common ancestry are stronger than those bonds between unrelated people (such as friendship).
Family (relative) bonds are sometimes only stronger out of obligation. Many people do not get along with or like certain members of their family, but will be there for them when the chips are down. For me, that’s what “Blood is thicker than water” really means. There is also a saying that goes something like, “friends are the family we choose for ourselves”.
What’s at the root of some family issues? Religion.