Not sure if you are saying your sis has MS, or this question is for MS? If its the latter, pardon my butt-in.
My sister used to take dexamethasone for Crohn's disease. It is WILL cause sodium retention, so she needs to really watch her overall sodium intake. It will probably also cause her to really lose potassium via urine, so electrolyte balance is problematic. Usually the doc will advise extra potassium rich foods or supplementation. We found that a lot of the salt-substitutes were high in potassium, cheap, tasted good, and made other supplementation unnecessary.
I am a little curious though, have they already tried her on methylprednisolone or interferons? Dexametasone has soooo many negative sides it is rough to use. Watch out for depression, my sis really had it bad, and she is usually the eternal optimist!
Best wishes to your sister!