When someone you know and love is addicted to drugs or alcohol, it's a tragedy not only for the addict, but for the wider circle of family and friends. While it's natural to be embarrassed and to try to keep the problem hidden inside the family circle, you need to realize that addiction is a type of disease, and it should be treated as such. Just as you wouldn't expect to be able to cure a cancer patient without professional medical intervention, you shouldn't keep an alcohol or drug problem inside the family and expect that it will heal itself. It is very important to turn to outside help as soon as you are aware of the issue.
One of the organizations that can help you is the National Council on Alcoholism and Drug Dependence (NCADD). They have chapters across the U.S. and can offer advice and contacts in your local community. Other major treatment centers, like the Caron Foundation and the Betty Ford Center, may also be able to assist. You probably need to start with an intervention, and you’ll need to find a trusty and appropriate professional to work with the addict and the family.
There are different approaches to both intervention and treatment, and you'll need to learn what's available and what's going to work best for the people involved. Some people think it necessary to have a "surprise" intervention so as to ensure that the addict will be present, but there are professionals who don't like to work in this manner, preferring that everyone knows up front what will be happening. Even if the addict is not present, it may be possible to involve the rest of the family in talking about ways to deal with the situation.
Each addict is an individual who needs to have a treatment plan that will take into account his or her particular issues and unique personality. The professional interventionist will probably be able to direct you to a rehab center or program that fits your particular needs.