Eating disorders are NOT about vanity, they are frequently a response to trauma, abuse being a common one and you may not even be aware of it at this stage.
I don't have an Ed but a related condition and at your dds age my mum didn't know I'd been sexually abused by my dad.
Nobody with a charmed life develops one of the most serious mental illnesses going. Something triggered it, not saying your fault but you may well at least be unaware of the trigger.
I think you have some odd ideas about how to help and support someone with an Ed. Have you sought professional advice on this?
How can I support her when she won’t admit she has an issue. What more can I do? off the top of my head and with no expertise on Ed I would say
Positive comments
Healthy attitude to food and weight generally around your home, think if that's really the case
Supportive of her generally when there are things stressing her out
Complimenting her on personality traits, when she does something kind for another etc
Sympathy (seems to be lacking to be perfectly honest)
Do fun things with her that aren't diet/exercise related
A 25 year old with a serious mental illness which physically impacts growth too may well be "immature" compared to others her age who are healthy. That's not necessarily her fault
I agree intimidating the younger dc and being disrespectful isn't on, and must be dealt with but given the Ed maybe carefully so?
She’s 25 years old. She isn’t a child.
Whatever age she is she will always be YOUR child though.
I'm 48, I still turn to my mum for support at times. Don't you?
The situation with her dad sounds pretty traumatic to me! I've a dd almost 20 with a very difficult history with her dad and it's definitely affected her. She has anxiety and depression, thankfully not to a very serious extent at the moment but there's been times I've been very worried about her.
Also why would having brothers and sisters be traumatic?
Comments like this show you have little insight or sympathy for your eldest.
However well handled (and to be honest I suspect it wasn't!) a new step-parent and then new siblings is a huge change for all children, but especially in the difficult teen years the older child, however well they MIGHT seem to be coping still needs support, reassurance they are still loved and important within the family and allowed and encouraged to discuss any negative feelings.
Geez even toddlers don't always react well to a new sibling!
You don't TELL them they're embarrassing though! Certainly not in a situation relating to self image when you know they have an Ed!
She wasn’t a small child struggling to find her place in her home.
No she was a young teen who may have felt she was being squeezed out!
I left home at 17 not because that's an ideal thing to do but to escape an abusive home. I've yet to meet anyone who left home at a young age without there being some kind of issue in the home.
You're very defensive and there's lots of denying type comments, the tone makes me think there is something you know or suspect and are trying to ignore.
Often with mental illness there's a "perfect storm" situation so not one thing in particular but several things happening at once that make the sufferer feel overwhelmed.
Mental illness generally, apart from a few of the more rare and extreme types come out of the wild blue yonder.
If you were to speak to my parents on mine you'd get the impression they did nothing remotely wrong, that it was a totally unpredictable surprise etc the truth is far from that.
Others I've met with mental illness it's been triggered by assault, burglary, bullying at school or work...
But there's usually something.
Few people lead a totally charmed life anyway.
You sound overwhelmed and struggling op yes, but you also sound as if you're building up to blaming your dd for this when she is but one factor and from the sounds of things not the main one, that seems to be your stbexh who isn't shifting and should be. That's an incredibly stressful position he's put you in. I reckon if he were to move out you'd feel a lot more able to cope with the other stressors in your life.