It could be Diary of Laura's Twin written by Kathy Kacer. It tells the story of a young girl who was given an assignment during her Bat Mitzvah to find a child her age who had something to do with the Holocaust. She befriends an older woman who gives her a diary which takes her through the life of this young girl.
But I think whom you are looking for are the twins Eva and Miriam Mozes. There is a web page devoted to them. www.auschwitz.dk/eva.htm
Eva Mozes Kor wrote books on the experiments and horror of their time in Auschwitz and founded the Holocaust Museum and Education Center in Indiana. Check out the site...amazing.
If you are interested in books like this, might I suggest a book called "The Devil's Arithmetic" written by Jane Yolen in 1988. I was introduced to this book i 1989 (my 5th grade year) and it still remains one of my favorite books of all time. I intend for my kids to read it someday so they can understand more about this terrible time in history. It is a very powerful coming of age story filled with courage, love, sacrifice, and understanding. Amazing. I hope you find the book you are looking for and maybe you'll pick this one up too.