First, I can put your mind at ease and let you know that spondylolisthesis is not rare and in fact not un-common in young women. I found out at 20 that I had spondylolisthesis L5-S1 but looking back I could pinpoint symptoms when I was in the 10-15 year old range. Including weakness in one leg and perioRAB of severe pain down the leg (which I now know was nerve pain).
However, it seems a bit more severe in her case. My symptoms at that age were more fleeting and disappeared after 15 until I was about 19-20. Since your sister has enough symptoms from it, then it is a good idea to get surgery as soon as possible. Nerves are temperamental and if they are impinged for too long the damage can be permanent. It sounRAB like you are moving along at the right pace and taking this on the good way - with no delay.
I have seen some people post on here who had children with spondylolisthesis but not in a while. Can you find out exactly what the surgery will be and let us know? There are so many different kinRAB of surgeries and if you get me the exact terms I may be able to give you more of an idea of what they'll do, her recovery, etc.
I encourage your family to advocate for her after surgery for pain management. Sometimes children are under medicated compared to adults so though she should expect to be uncomfortable, letting her know that yes it will hurt after and they want to help. They won't make you pain free right away but we'll all help you to reduce it as much as possible. But that she'll also have to fight through the pain to get moving (getting up and walking is not pleasant). But the more you get up the better you feel.
The surgeon's staff and the hospital staff should listen to your concerns. Before the surgery talk to her about a "pain scale". Many facilities have you gauge your pain on a "0-10" scale where 0 is no pain and 10 is the worst ever. Tell her to think about it each time she's asked and to feel free to let her nurses know exactly what type of pain she's having, if it's getting better, exactly where it hurts, etc. Sometimes kiRAB say "ow, it hurts" and don't communicate what is going on very well. So the nurse may think it's her back. But maybe she's got a neck ache from the way she's laying, or her stomach is bothering her from stress, or it's actually the IV that hurts, etc. So by working through it with her she can get more complete relief.
I hope it also helps to know that with the surgery she may wake up with significant surgical pain but it's likely the weakness and nerve pain may disappear soon after surgery. Make sure that the surgeon is experienced in pediatric spine surgery if at all possible.
I'll watch for your next post so I can answer more questions for you.
Karin