I bought this book yesterday on impulse and just finished reading it this morning (it's a quick read).
The author, Molly Bruce Jacobs ("Brucie"), tells the story of sobering up at age 38, and meeting her sister Anne for the first time. Anne was born with hydrocephalus and moderate retardation and placed in an institution at birth. Brucie didn't even learn she had another sister until age 13, when her father finally revealed the secret to her and her sister Laura (the "secret girl" was actually Laura's twin).
It continued to remain a shameful family secret, with no family members ever visiting or contacting Anne, even though she lived only a half an hour away. The family had more than enough resources to keep Anne at home, but as the author explains, the parents' seemingly horrifying treatment of their own daughter was actually considered to be a "normal" decision at that time (the 1950s).
As Brucie forges a relationship with her lost sister, she gains startling insight into her own problems, discovering that her own lifelong feelings of inferiority and rejection are what fueled her alcoholism. She realizes that she has a surprisingly strong, natural kinship with this sister that she had always assumed was so completely different from herself.
Brucie is now a painter; I looked her up online and saw some of her paintings, including this one at left (called Oranges).
I liked this paragraph (describing her thoughts at age 13): "I was relieved that Anne would remain a family secret. For I was still wrapped up in the naive belief that I was something of a princess, destined for reward and success, and entitled to glamorous pleasures in a world that did not accept the likes of Anne. A world I now understand as displaying a weakness, or at best an apprehension born from ignorance, in its custom of hiding people like Anne behind institutional walls."