Indeed. Ive read that Ted Bundy's mother didn't believe he did anything. Even when he admitted it she had a hard time accepting it. A lot of parents can be wilfully blind to who their kid (whether still a child or an adult) really is.
Yes sometimes they can but also it could be that they can't see the forest for the trees too. When you know someone and are accustomed to them and their peculiarities, you stop really questioning it. At times I think what we might pass off as a 'quirk' amongst those that we know (and usually a harmless one at that) may read as more of a 'warning' to an outsider.
Sometimes you literally just can't see it. It can be hard to look at an inside situation with outside eyes.