Physical therapy is helpful if the therapist is good understands scoliosis.
Your grandaughter's growth plates are fixed, so she could have surgery if her curve is greater than 45 degrees.
There are other minor changes that can reduce the advancement of scoliosis. https://treatingscoliosis.com/scoliosis-dos-anddonts/#:~:text=Repeated%20extension%20of%20your%20thoracic,often%20causes%20rapid%20scoliosis%20progression.
I (may) have adult onset scoliosis, but have been told by an orthopedist that the one thing preventing the scoliosis from advancing is my fondness for chiropractics.
I admire your wading into a treacherous situation with your daughter. You are a very good grandmother. And you have an experience that you don't want your grandaughter to have.
I hope your daughter will heed your suggestions because you have very little power here. Scoliosis is broadly misunderstood, though the two opinions may be accurate.
You might explore with your daughter the struggles (tortures really) you've had with scoliosis so she is awed by the power of your motivation.
She might let you buy your grandaughter a new mattress.
There is a bracing system, even for adults, called Clear Scoliosis. There is also the schroth method a physical therapy technique that is usually too expensive.
I love your involvement as much as your daughter probably doesn't.
wishing you luck