I would disagree with the Spyderco Cricket, especially if you are thinking about self-defense. It's a great cutter, yes, but it would be impossible to hold onto and its not made with a piercing tip, which is really what you're concerned about in a self-defense situation.
I would highly back the recommendation for the Waved Delica :thumbup: and the HAK Why? The Waved Delica is small but not wimpy, it should easily fit into the itty-bitty pockets that they put on girls' pants, easy to draw in a stressful situation, 3" blade (usually legal and unintimidating), great for utility as well.
The HAK on the other hand is good if you are concerned about lack of training and reluctance to use deadly force in a date-rape situation. Personally, I have no sympathy for guys that try that sort of thing and could care less if they were gutted (and I'm sure you probably have similar feelings, she being your sister and all) but you don't want there to be a hesistation factor for her. It's small, so theoretically she could just get them in the thigh and run; it is also literally effortless to retain, no muscles involved; and as for ease-of-use, well, anyone can make a fist. I think also as a fixed blade you don't have the worries that you would have with a folder, and Frontsight (the maker) has made an effort to design a number of carry options especially for girls.
As for laws and rules and legality, I think as knife knuts we all want to be responsible with our equipment, and I completely support TKC in her statement.

That said, however, I find that LEO types are MUCH more forgiving about girls carrying knives and MUCH less likely to give girls a hard time for carrying them. Maybe it's because there's isn't as much of a tendency towards "mall ninja" behavior? I dunno.

So that's something to think about.
The BM Minigrip is also a favorite among girls.