Why do family members protect the dysfunctional ones?
Because they're dysfunctional too.
Read up on Stockholm Syndrome for an extreme example. But also consider that most people aren't really very nice, and live along tribal lines. Us vs. Them, and the Us is very, very small.
The illusion of sophisticated broadminded intelligence in the world is utterly dependent on the power not going out.
Because they all have a role in the family script. Casey was but one part of a whole.
Casey’s only one part of the dysfunctional family. They protect each other because being tight-knit and protecting The Family is more important than pretty much anything else.