I disagree, if i remember correctly, Shallan had the thought that she didn't even think to summon her shardblade during that attack. To me this indicates that she has not had the blade long enough (or used it enough)to become used to it.
Take what I said above, and add in the following scenes:
the scene where she calls the shardblade "the fruit of her sin"
the image of her father lying facedown on the floor, dead, surrounded by blood
the fact that the fabrial was found in his breast pocket, sheared in (3?) places
the scenes with her disabled brother
and I emerge with a picture something like this:
[Cat's vision of how it happened]
Shallan's father is abusing her brother (possibly trying to kill him in a blind rage?) when Shallen walks in, grabs a sword off the wall, or a knife off the table, (or something to that effect) and rushing to her brothers defense. By this time the brother is unconscious, the father turns around as Shallen screams and he is killed (possibly on accident) by Shallen. In addition to killing her father, the fabrial is "sheared" at the same time. The father falls face down and his shardblade appears beside him, Shallan picks it up, bonding it. She hears her brothers coming and drops the blade, causing it to dissipate. Thus the brothers don't know about the blade.
[/vision]