Dear Thomas_1980

You have described a non capsular spinal limitation...I agree with the other two comments above...if a teenager is having issues with Wikipedia reference-linkscoliosis, this might be due to rate of bone developement...scoliosis however does not always result in pain...what side is her scoliosis convex towards?
the non caps presentation might mean a dysfunction in a particular vertebra or group of vertebrae... causing excessive movement one side greater than the other...three week onset is quite recent...how did it start?
any aggravating movements? flexion. extension, rotation, combination movements, where is the pain? how does it behave morning, during the day and at night? what tests have been done? what happens with upper limb movements? does this pain go anywhere? what relieves this pain? is it constant? what activities is she involved in...sports etc? any pain killers? what kind? does breathing or coughing influence her pain? whats the description of the pain? how much is it...is it localised or diffused/non specific

are you getting any joy from your input so far?

just at the top of my head, I would suspect that if the issue was muscular, with regards the limitation...shouldnt it be restrictions on the other side that would make these movements poor?
The traps... attach to spine, scapular spine..if the upper traps are tight...my guess is that this would affect neck movments to the other side, flexion and upperlimb movements on the same side. this shouldnt influence the poor movements in the thoracic region on the same side...the question however is why are they tight?

Taping: what are you taping and what is the aim? to correct poor sitting posture...how do you mean poor sitting posture...

Being a growing teenager, i would take care in using my manual techiques...but i think before any treatment advice can be given, a more detailed assessment is required to be honest....

Sorry i havent been of much help...lol