How Can my 3 year old Sister be cured from Severe Urticaria?

You don't "cure" urticaria, because it is an allergic reaction to something (allergen). Urticaria is a natural response. What you do is find out what is she allergic to that is causing the urticarial reaction. Then, you either stop the exposure to the causative agent or you sometimes take allergy shots to desensitize to the allergen. Medications can keep symptoms under control (antihistamines).