Dysthymic Disorder. Dysthymic disorder is a milder condition than major depression.The disability is less. It is not bounded by a distinct cause or time of onset rather being chronic in nature, more like a personality trait. It is characterized by low moods, pessimisstic or negative trend of thinking, lack of enthusiasm, always seeing the glass half empty. Anti depressant drugs may or may not help in this disorder.
Dysthymic disorder. Is a chronic depression that means the person has felt down more days than not over the past 2 years (1year in children and adolescence). And, there can't be a major depressive episode before or during that period. For your sister, it means she needs treatment and your love and support.