You know, I had the same situation (wasn't my sister, was my wife) but she wanted a handgun, something she could use in the house and take with her on road trips as a glovebox gun, because she even realizes that I will not be around all the time (like right now, sitting in the middle of Iraq on my second deployment), so I got this idea to buy her a revolver, don't have to rack the slide, just point, aim, and pull the trigger. Bought her a Taurus Model 85 Ultra-Lite, needless to say she hates the gun, but she loves my XDm 9mm (sorry, paid $615 for the gun, she can't have it).
Long story short, best thing you can probably do is take her out to the gun range and let her try out some handguns, alot of gun ranges have pistols that you can rent, rent one and try it out. All in all, let her pick the weapon, this will put confidence in her weapon, then the more that she shoots it, the more her confidence will grow if you know what I mean. I would say to be by her side so you can give advice to her about a weapon to help her in her decision, don't make the decision for her, just help her out. In the end, she should make a good choice.