Loyalty and Terpitude

What causes family loyalty? What I'm asking is this: Why do we put up with behaviors from our family members we wouldn't put up with from a friend?

Every so often I repeat a similar situation, by deliberate act of will, and during and after the event I think about the above question. It brings me to this quote:

Insanity is doing the same thing, over and over again, but expecting different results. -Albert Einstein

Here I am, in the same depressing, miserable situation wondering why I even bother. And as I've gotten older, I've bothered less. Perhaps, someday I'll stop bothering at all.

Terpitude is defined as:

Inherent baseness or vileness of principle, words, or actions; depravity.