@fify-- It definitely sounds like she has circulation problems and she should see a doctor as soon as possible!
I have Reynaud's disease and the symptoms you mentioned about your sister are the same as my symptoms. Reynaud's usually shows little or no symptoms during warm weather. The symptoms come about in the cold. My hands also change colors in the cold and I get pins and needles and numbness.
But you can't be sure until she's diagnosed by a doctor. There are other nervous system disorders that may have these symptoms. So have her checked out!