I just (as in just a couple of weeks ago) learned to ride a bike. I am 44 years old and probably the only adult I know who didn't know how to ride a bicycle. For some incredible advice, check out my post from a little while ago. I got lots of really terrific advice and useful information from the awesome people here. I don't know how to insert the link to the post, but it's only a little ways down from yours. It's titled "Riding Tips For The Extremely New And Klutzy". Last post was dated 6/8.

Some personal experience tips:
1. Try teaching her on a Commuter type bike or if none are available, then a mountain bike will do. (I learned on a mountain bike.)
2. Try lowering the seat so she can put her feet on the ground. You can raise the seat as her confidence grows.
3. Let her coast on a very gentle downward sloping area to get the feel of balancing on a bike. When she's comfortable, she can pedal. Just remember that the lower the seat is, the more challenging pedaling will be, so you're in a bit of a catch-22 there. Raise the seat as soon as possible so she won't get frustrated with pedaling in such a compressed position.
4. Some people suggested trying to ride on a grassy area before riding on asphalt or pavement. I did it in a parking lot that had little traffic. Just a reminder that if she does try riding in a parking lot, make sure she keeps an eye out for cars. I was so engrossed in trying to stay upright that I almost got nailed by a very large truck speeding into the parking lot.
5. Have her try out the rear brake before she actually gets going so she gets a feel for how it works and how much pressure to use. I personally avoid the front brake right now - especially since grabbing it by mistake on my first ride...which resulted in a particularly spectacular unintended dismount over the front handlebars. I did a full rotation in the air and landed on my back. The Russian judge gave me a 5.0...
6. Have her read the article on steering with your hips (extremely helpful). It's linked on one of the responses in the thread I started. That advice helped me tremendously in learning how to steer and turn. Before that I was steering with my handlebars and was all over the place.

I still have a lot to work on, and I am obsessed with getting on my road bike...I only ride my mountain bike right now since I tried to ride my road bike and crashed. I have been riding a little every day (about an hour) and trying to master the technique of not falling down. It's been frustrating, a little painful (my legs are bruised, bloody, scraped and scabbed over), but mostly incredibly fun. I'm sure your sister will love it!

Good luck!