My sister went to doctors for a long time, she has psoriasis, but no medicine completly cured her, it disappears for a while then returns , can u help?

Varies on severity: Small patches may respond well to various potencies of steroid cream, vit.D-like drugs, or vit a-derived meds. Larger areas of involvement may require immunosuppressant therapy like Methotrexate or new immune-modulator drugs like infliximab, etanercept, etc. The latter drugs have many side effects and precautions; so you should be under the care of a specialist in those.

Psoriasis treatments: Psoriasis is a chronic disease that goes through exacerbations & remissions during one's life. There are many medications that treat psoriasis, from topicals that help control psoriasis confined to small areas to oral or injectable medications for more extensive skin involvement. Unfortunately we don't have a cure yet, but the disease can be controlled to give a patient excellent quality of life.