Gilenya is prescribed for relapsing-
remitting MS. From your limited description of current
symptoms, with no historical perspective, I would guess your sister is secondary progressive. But it's only a guess. Perhaps that's why the four earlier treatments failed, because they are for relapsing-remitting. But we don't know if/how they really failed, how long each one was used, etc. And there is no specific drug called "Interferone" ...