Hi Olivia and welcome to the board
. My first question is to ask if you are going to address the anorexia and try to come to terms with it? That would be the first step in making repairs to your bones. You are very young to have severe osteo, but are young enough to make some positive lifestyle changes that could help you immensely. Improving your health in general and getting the needed help to do so will help quite a bit. Talk to a physical therapist who can give you all the dos and don'ts in regard to exercise, movement and staying fracture free. A few things that you want to keep in mind are to bend from from the hips, lift with bent knees, avoid twisting motions and be aware of how you move. You've had this for quite a while without breaking anything and lead an active life, so now that you know just be aware.
As for telling your parents just sit down with them and tell them what you were told. They probably already know about the anorexia and if they have done reading about that they know about the risk to your bones as well as other health factors that come with it. Be honest and keep your cool when discussing things. I have 2 grown kids so i know how difficult this will be for you, but your parents love you and will be understanding of what you tell them.
Another thing you can do is to read The Myth of Osteoporosis by Gillian Sanson. Its a very informative book that will help you understand osteo. Its a different perspective from other books that will help you get over the shock of the news. Its been very helpful to many of us here on the board. Keep us posted on how you are doing. There are a lot of "mom" types here who will be glad to help you. take care...phyllis