I don't have a little sister (and I'm no expert), but I'd like to point out that OC spray only gives you some time. Enough time to get a head start running, but make sure you think through the "what next" part. This makes it a particularly poor choice for the elderly for example - IMO.

Also, make sure you select a quality OC and understand spray patterns and select one that is approprate. I understand the Fox Labs spray is good and think a 2oz size is a good tradeoff of size vs capacity. Their "flip top" lid version prevents accidental activation.

Typical spray patterns are stream, cone and foam. Stream goes the farthest, but is harder to hit the target. Foam is for indoor use since otherwise it could get the user as much as the assailant. Cone is in between and probably the best compromise.

I'd also be fairly leary of using it against crowds. The problem is as I was saying is that it only buys you time. Some folks are less prone to it as well. The chances you hit everyone is pretty low. You add up all the things that can go wrong and the chances are that at least one person isn't going to be incapacitated for one reason or another and they are going to grab you. The other downside is that being sprayed isn't going to improve their mood any.

Another option to consider is a collapable baton, like the Monadnock Autolock. Perhaps in conjunction with the OC spray. I haven't handled the Monadnock Autolock, but I have the ASP batons and didn't realize they rattled - a serious drawback IMO. The Autolock shouldn't have that problem. 21/22" is a pretty good compromise between concealability and useful length. It would be nice to have something more like 26", but it's probably not going to be as easy to conceal.

In any case, you are going to want to pursue training, and I have no idea about the legal issues in your area.

For example, in WA, the way I understand things (and I'm NOT a lawyer!), you are probably OK to carry a baton as long as you don't use it to intimidate anyone. So, whipping it out casually wouldn't be OK, but keeping it hidden, and only deploying it if you were attacked probably is fine. Did I mention I'm not a lawyer? And of course in many places, it's pretty much illegal to defend yourself in any way.

And of course, it sounds like you can expect to be sued in any case.

-john