Why it is so difficult to think critically about family members behaviour? It seems to me that some survival mechanisms working in us make the act of analyzing family members behaviour very difficult. We tend to perceive the family's behaviours as normal, even if there are not. In my experience, we must achieve a high level of detachment from the family to be able to think critically.

So my question is, what are our mechanisms that makes it so difficult to think objectively about family members' (aberrant) behaviours?
Short answer
Millions of years of evolution have resulted in increased empathy towards family members.  

background
I may be taking a big leap here, but I think the roots of the family-related tolerance may be ingrained in our very genes.

In an interesting article by Tomasello (2018) he hypothesizes that collaboration was among the key features that resulted in the separation of the homo lineage from the apes, starting around 2 mln years ago. Homo heidelbergensis (400k years ago) is believed to have actively collaborated when foraging for food. This collaboration became compulsory, as individuals became dependent on one another to obtain their daily food. Tomasello hypothesizes that morality and empathy were born out of this interrelationship, which helped to sympathize and help their partners. The partners learned to cooperate in hunting (one would chase, the other spear the animal). A "we is greater than me" kind of morality was born.

Then, 200k years ago, modern homo was born, an era characterized by competition between small, closely knit groups for food and other resources This meant that members of such a group would have to collaborate and protect each other against invaders. Often (but not always) there would be kinship involved in these small groups ('tribes'). Tomasello continues that this collaboration eventually resulted in the development of culture, but that's outside the scope of this answer. 

In all I think I would conclude that millions of years of evolution have resulted in increased empathy towards family members.  

Reference
- Tomasello, Sci Am, September issue 2018