I don't think it is a Shardblade. Why would you put a Shardblade in a safe to keep it safe when you can just turn it to mist. I think Shallan's family was attacked by a group using a Shadeblade. During the attack Shallan walks in frightened picks up a Soulcaster( cause she has seen it used before and knows it's powerful) and kills them. After the deaths Shallan claims the blade by being curious and touches it. Her father seeing all this, scared out of his mind for his daughter, put the soulcaster in the safe calling IT a monster cause he doesn't want to blame his kid. Her father then becomes super protective of her cause he doesn't want her to remember the events and is scared of her a little.