Well, RLS is only part of my sleep issues. Like I said, I also have OSA and am borderline Narcoleptic. The sleep study that I had done, and diagnosed me with these things, showed me falling asleep in about 5 minutes (which is borderline Narcoleptic) but waking up an average of 9 times an hour. So I know how your sister feels, having to go days, or longer, without any descent sleep.
OSA is the number one leading cause, of excessive daytime sleepiness. Narcolepsy is the second, and RLS is the third.
I don't know what your sister has tried, but RLS is sometimes caused by underlying sleep disorders...so finding, and treating, those can help reduces RLS. Also...losing extra weight, decreasing caffeine, alcohol, and tobacco can help...or adding a regular exercise regimen (however exercise can sometimes aggravate it more), she could also try iron, folate, and magnesium supplements because RLS can be caused by those deficiencies. Some people find that warm baths and massaging their legs, before bed, can help reduce the symptoms, too.
Those are, really, the only possible treatments, that I know of (other that giving her something, like the Codeine, to knock her out). Personally, though, nothing works...boy, do I feel her frustrations!