Well, blood is thicker than water.

I think it's generally true simply due to the fact that familial relations are, for most people, the longest we'll ever know. On top of that, social norms strongly encourage the formation of and loyalty to the family unit in various ways. Unsurprisingly, the strongest bonds are often between family members. For most people.

But that doesn't mean the bond is unbreakable, particularly when family members are abusing the idea of "family loyalty" for their own gain. In doing so, they corrupt the very notion of "loyalty to one's family."