Hello,
That's so nice of you to try to find something more nutritious for your sister. Stevia is considered one of the best ways to sweeten things, and you can bake with it. I have a cookbook by Ray Sahelian and Donna Gates (Body Ecology Diet - Candida self-help book) that's called the Stevia Cookbook. It has all kinds of recipes in it from ice cream to pancakes, cookies to kefir shakes. Let me know what kind of recipe you would like, and I'll type it out for you so you can try it.
I've seen the stevia from Trader Joe's. It's the processed kind of stevia, and most people do use the processed form. Trader Joe's has lactose in it, which makes it a little bit different, but as long as your sister isn't lactose intolerant, I'm sure it would be better for her than regular sugar.
Let me know, and I'd be glad to share some recipes with you.
Betta