Don't regard this as a family disgrace or a subject of shame.
Mood disorders are biochemical in nature, just like diabetes, and are just as treatable.
Don't nag, preach or lecture to the person. You may only increase their feeling of isolation or force one to make promises that cannot possibly be kept. It is possible to create this impression without saying a word. Since persons with mood disorders are not in control of their affliction, this approach only increases guilt. Avoid any threats unless you think it through carefully and definitely intend to carry them out.