Dear Felia,
On two different occasions, once in 2002 and once in 2004, I had a ligament/tendon problem, first in the right hand forefinger and then in the left hand forefinger.
The first time it took from January until May before I was able to play again, and then only very light things and no heavy chords; by August it was 100% right and I have had no problems since. I didn't bother to go to a doctor, because it was clear that it was a ligament problem and not arthritis or otherwise caused. In fact I know the origin exactly: repeated practise of two lines in Franck-Bauer's Prelude, Fugue and Variation which required (or so I thought!) stretches of the right forefinger. So I figured I just had to wait.
The second time it took from February to June before I was able to play properly. This time I did go to a doctor because it was in the LH knuckle and I wondered if it was arthritic or otherwise caused. He did an X-ray and concluded immediately that there was no evidence of arthritis at all. He said after a torn tendon it usually takes SIX TO NINE MONTHS for healing to occur for a person of my age (40s).
From the symptoms you describe, and the doctors' diagnosis, it seems your sister has a standard torn ligament injury. I would suggest she rest the relevant fingers/hand and work on the right hand, sight-reading skills, theory study, extending her knowledge of repertoire and music history. The good news, as far as I can make out, is that there is no permanent damage and her hand will be back to normal, 100% recovered, in a few months.
I would also suggest, as Hmoll did, that she take a careful look at her technique and figure out if there is something that leads to injury.
Best of luck,
Peter Moll