I wonder if maybe Shallan doesn't know what her Shardblade is and how valuable it is. She never refers to it as a Shardblade. She has no background in military or fighting matters and might not know enough about Shardblades to recognize it as one - especially if her father having a Blade was a secret.
Maybe the events happened something like: Shallan's father dies, his blade materializes, Shallan grabs it, but later lets go of it (perhaps in horror), and it disappears, as Shardblades do. If no one witnessed these events, perhaps no one else saw the sword at all. Or perhaps the only people who saw it also failed to recognize what it was, and may have thought it was gone for good when it disappeared.
Shallan's brothers never say anything indicating they know she is the one who killed their father. Maybe the two were alone when it happened? Or just Nan Balat was there, but he was too badly injured to see what happened? In the viewpoint we have from Nan Balat, he thinks of Shallan as "Shy, quiet, delicate"; he doesn't seem to regard her the way you would expect if he knew she had killed someone.
Brandon has said previously that picking up a Shardblade confers you with the knowledge of how to use it. This is how she knows that it takes ten heartbeats to summon the blade- which perhaps is something Shardbearers keep to themselves. I don't recall it being mentioned, but I haven't done a close read of the book yet.
I do like the possibility that she doesn't know much about her blade, given that only the men seem to talk about Shardblades and Shardplate, but for now I think it's purely a guess.