If your attacker is within 21 feet, and has even the slightest element of surprise, you will not be able to draw from a concealed location and hit him. Reality sucks.
I'm not saying that carrying concealed isnt worthwhile, but if somebody REALLY wants to get to you, they will.
Unless you sister is willing to put in a serious commitment to training with whatever kind of weapon she chooses to carry, her voice/hands/situational awareness is a more appropriate tactic.