Well,dinkleboo, I think you're wonderful. It is ,on the face of it, absurd for social services to take your sister's word. They ought to know that the last person to admit alcoholism, the most devious person about it, and the person best able to pass themselves off as functioning normally, is the alcoholic.
All I can add,as much for others who may read this as for you, is that if you still find yourself stressed by a family member who is alcoholic, there is a group, a branch of AA, created for the very purpose of helping people in that situation. It is called al-Anon and may be found via the AA.