How terrible for your little sister! Not trying to come across as pessimistic, but it's going to take forever to grow out again.
I don't trust salons with my curly hair (which looks like Misty's but golden-brown. One of these days I'll get around to posting a picture). Anyways, I went in about 18 months ago to get it cut because it was all one length and tangling. I figured taking off the weight would get rid of the tangles, so I asked the stylist to take it off at the shoulders, and maybe put some layers in it. She wouldn't do the layers, saying that they would have the feared "triangle-head" effect, but leaving it all one length at the shoulders meant that I had the same amount of tangles as before the cut - and much shorter hair. Imagine my frustration when the cut didn't fix the problem, only took away the length I loved.
So, following the Curly Girl method (the book Nighean mentioned), I took it upon myself to give myself layers. I've gotten better at trimming since that first cut about 15 months ago (although one side of my hair is currently shorter than the other due to a haywire trim in June), but you shouldn't have any problems cutting your sister's hair since you'll be able to see all of it.
To cut curly hair, make sure it is detangled and COMPLETELY dry before you start cutting. I've cut mine before when it was a little damp, and cut too much off consequentially. Then, you cut in the bend of each curl, so you'll end up with a bunch of little ringlets all over the floor. Cut each curl separately (a daunting task, and one my mom won't try just cause it takes so long), so that when you're done they all fall the way they want
to fall, since that's how they're going to fall every day. Don't pull them tight, just pick it up and let it be relaxed and then slide the scissors around the curl and snip it off. Usually I pinch a curl right above where I want to cut, lift it up so it's relaxed, and then snip it off right below my fingers.
You can cut layers in it too, but don't cut them straight across or your sister will have a bunch of "shelves" in her hair. Instead, you take a curl and cut it, then cut the one next to it a little shorter or a little longer, and just go around the top level like that. Then you can cut the middle level(s) the same way, so that the curls are different lengths but none of them (except the bottom level) is exactly the same length as all the other curls on the same level.
The first time you cut it might take anywhere from an hour to two hours (my first self-cut took 1.5 hours, but now I'm a little lazier about it and only trimming/ maintaining the levels so a trim will take about 30 min for me). But plan for a significant amount of time. And if you mess up a few curls, don't worry about it. Curly hair is very forgiving of mistakes and hides them well. I'm the only one who knows that one side of mine is shorter than the other side, so I don't worry about it. I'll have to fix it eventually, but for now I just leave it be.
Hopefully this helps, just post if you have any questions!