Hey @Dinnerisburnt, just wanted to send you a hug. It's really overwhelming at the start of this when you're trying to understand what to do and how to help. It's a daunting thing to have to deal with, but your dd won't be able to easily change, most of the work will come from you.
For young people the best evidence based treatment is called family based treatment (FBT). There are three phases to the work:
Phase 1: weight and nutrition restoration. Parents take over all control of cooking and feeding - the young person is not involved in choosing, cooking or plating food. They must eat all of what is given to them. This can be a very hard task indeed, and the person usually resists this. Parents are encouraged to see the eating disorder as a separate part of the person, so when they are very upset or angry to make sense of this as the eating disorder fighting back, not your dd. During phase 1 the person also learns to cope with 'fear foods'. Medical help is often needed in this phase as if the person has been eating very few calories per day, initially they are at risk of 'refeeding syndrome' because the electrolytes in their body become unbalanced when they start to eat more. After the risk of this has passed, the person eats 3 meals and 3 snacks every day, with increasing calories, as well as a lot of fat especially animal fat. This helps a malnourished brain recover, and can take a long time, at least one year but likely more.
Phase 2: once weight is restored (and will probably need to be higher than an initial target set) the person and their family gradually change the balance of power in terms of food and eating. The young person slowly takes over control of some parts of their daily menu. The goal is for them to choose foods that will help them to maintain, or probably gain weight (young people need to continue to gain weight until early 20s).
Phase 3: might involve psychological therapies to help undertake and treat any underlying issues which contributed to the development of the eating disorder.
Actually I realise I don't know what phase 3 is, I've been so focussed myself on phase 1!!
If you haven't already got Eva musby's book that is a very good place to start, and she also has lots of short ish YouTube videos. BEAT is a charity who have lots of info and run parents groups - we did the 'developing dolphins' group recently which was really useful.
And this group of people here are amazingly supportive and knowledgable. I know I'd be in a much worse place without everyone here, so do use us as a resource / sounding board.
@Eyelashesoffire my ds is about that age too. I've tried to be as honest as possible - told him that his sister's brain has a part which is poorly, and this part is bullying her to not eat. I've said it doesn't make sense, but his sister can't help how she is behaving at the moment. I've said it won't always be like this, and she will get better, but at the moment it is very hard. I've said that this is what families do - they look after each other when one of us is sick or not doing well, and everyone needs help at one time or another. Then whenever there is a particularly shouty day I make sure to check in with him and just acknowledge that he's probably feeling upset, tell him I'm feeling the same. Last week we talked about how to make sure he has things which make him feel good in his life - sports clubs and friends and hobbies at home.
It's super hard, I feel like I'm clinging on with my fingernails to keep everyone ok, and failing miserably with quite a lot. He seems to respond well to just a completely honest conversation about how hard it is.
We have had a grim weekend, although with one little ray of sun - my dd got her period after not having one since late spring! I'm trying to hang onto that as a positive about her body recovering, although her mental health is very low just now...