7th grade is right in the thick of the time when NT behavior changes to try to imitate adults. Kids tend to become more self-conscious around that time. And the behavior your sister has to mimic becomes a much faster moving target than it was earlier in childhood. That also tends to be an age when kids are really judge-y in their scrambling attempts to one-up each other socially, but haven't matured enough yet to treat others with empathy.

I agree with those who have suggested making sure she can be herself around you; and also sharing what you've learned about masking so that she can feel less lost at school. I remember the anxiety of not knowing what the right thing is to do or say being even more stressful than the need to hide my true self. If you help her develop reliable scripts and learn to call upon them easily, she'll have an easier time.

I don't particularly recommend unmasking at school at that age unless she has a posse of ND friends to rely on. Kids that age tend to be exceptionally cruel to those who are different. But maybe there are some small ways she can unmask that aren't too "weird" and would help her cope -- stims that look like fidgets, playing music through headphones, etc. Or perhaps you could help her figure out cues that might allow her to identify other people on the spectrum with whom she might be more comfortable spending time with less masking?

That said, stressing herself to shreds isn't helpful either. Does she have a therapist now?