I don't really know what's going on in the heads of other people, but I personally prefer to err on the side of caution. If I consider all other possibilities (that I can think of or I read) and a family member ends up being guilty, nothing is lost because I'm not responsible for proving it, arresting the person, prosecuting him or anything else.
On the other hand, if I decide a parent is guilty and just spend my time finding ways it could have been done by that person, or where proof that the person did it could be found, and the person is innocent, I've added to the burden of a grieving parent, and possibly helped to permanently ruin his/her reputation by things I may have dug up and posted about him/her. I'll choose the first option every time. Not everybody will, and that's their choice, but I think I deserve as much time and bandwidth for my theories as anybody else does. MOO