Welcome to the site. It is devastating to watch a loved one throw their lives away to addiction, though there is nothing anyone can do to save an addict unless they truly want to get sober for themselves and to take the actions to make it happen.
There is no way to force someone into sobriety, and even if you were to get them into rehab, they almost assuredly would go right back to drinking the second they get out.
The best thing you can do is to walk away and leave them in your prayers that one day they will see the light and get help. There also are alanon meetings for support for yourself and the family and friends of alcoholics forum on this site worth looking into.