If it 'feels' unnatural, it could be she has been hunching to feel closer/more similar to her shorter friends, and now is just accustomed to standing like that - so standing up straight doesn't feel right. If she's not particularly athletic, her upper body muscles may be too weak to enable her to stand straight easily. So perhaps a mix of psychological or physical reasons.
Ideally you want to get to the point where slouching/hunching is the posture that feels unnatural to her.
My partner hunches and it gives him back pain and looks unattractive, but he refuses to do anything about it... it's such a shame. It's so good you're trying to help with this now rather than just leave it to her to figure out as an adult.
You could gently introduce some tall female role models - whether athletic, philanthropic, artistic, whatever. (see: https://www.reddit.com/r/tall/comments/y5voj/tall_female_role_models/). Seeing photos and learning about tall women who are inspiring - and own their height with good posture - may encourage her to want to walk tall.
If she isn't suffering pain from stooping and doesn't consider her hunch a problem, it may be a challenge to get her to just stand up straight by remembering to. Physio is only as useful as the time you commit to it outside of the sessions - it's pointless otherwise. So you could book a single session with the physio and make sure you get as many exercises and tips as possible, which you can then use to follow up yourself ensure your daughter works on them every day or two. She clearly isn't interested in standing up straight herself so you need to find a way of making it meaningful to her. Maybe introduce some kind of reward system by finding a way of measuring progress?
According to a quick google, it takes several months of pilates (and I imagine very regular/frequent sessions) to improve posture. Whatever you do, it might not be a quick fix.
You could start her on gymnastics, ballet, weightlifting (with guidance). Although pilates and yoga would be good for posture, they might be a little boring for a child. Doing slightly more athletic pursuits which have a strength element would promote good posture by building strength in the muscles that improve posture. Even cross-country running coupled with circuits (to build upper body strength) may help. If she's not a sporty child and doesn't like the idea of starting new pursuits where people could be watching - and it takes time to get better - get her into hiking, scrambling and rock climbing.... and get involved yourself!
You could get her to walk around the house for 5 minutes every day with a book on her head. And whenever she's watching TV or scrolling on her phone (if she has one), mandate that at least 5 minutes has to be done stood up or seated properly, again with book balanced on her head. If not, no TV and no phone.
Book balanced on head while eating too, to understand how sitting up properly at the table ought to look. Maybe you could both/all do it and make it a game. Even if it's for just a few minutes per meal, it might start reminding her to sit up straighter. After a bit of practice she (you all) can just imagine the book is there, and do pretend balancing.
If you want to go the tech route, there's a little electronic tag thing you can buy, it sticks to the centre of your upper back and buzzes whenever you hunch, reminding you to straighten up. Last I looked they were about £80. But I think this is pointless without other more functional strength type strategies in place.
You could also (coupled with athletic/balancing a book pursuits to improve her upper body strength) get her into something like public speaking or singing, which requires confidence, poise and good posture to ensure you breathe and articulate/project well. It's much more compelling giving a speech or singing when you stand up straight and exude confidence.
(On a separate note, and a rhetorical question. How is your posture? And that of any other adults your daughter regularly interacts with? I imagine it's good as her poor posture is something that bothers you.).