Sister1,
Welcome to SR.
I wouldn't recommend this, but if the boys have made up their mind to do this, there probably isn't much you can do about it. Until your sis is ready, they could move her from one coast and back again and it wouldn't help.
I know you're concerned and want help for you sister, too. Best thing I can recommend is finding an Al Anon or Nar Anon meeting in your area. You can go to those meetings and learn about what would be the best way to handle your life which happens to be affected by the disease of addiction.
Hope to see you around here some more.
Hugs,
Hangin' In