Gosh... That's all shades of fucked up. Pretending you have an eating disorder is wild... I feel like your sister might be my mother. Everyone who has ever met her agrees she has a narcissistic personality disorder - even those that hate throwing around labels and diagnosing people based off cod-science social media listicles. Combined with crushing insecurity and wild over-sensitivity, and it is a lot to deal with.
My mum is losing weight at the moment through a GLP-1, but used to manage diabetes, not for weight loss per se. She has always been morbidly obese, for as long as I can remember, and HATED her appearance, so you'd think the weight loss would be welcome to her. She's doing well and I'd say she's about a size 20 now.
Except she is framing it as a medical emergency - the weight is just falling off her, she doesn't know what's wrong, she has no appetite, look at her, she's just becoming so THIN... Yes mum. It's because you are effectively on mounjaro. This is what happens. This is the side effect. She's being monitored closely by the GP for an inflammatory arthritis condition at the moment and has regular blood tests. There's nothing in her blood to suggest that she is dying of something that causes unexplained weight loss. The weight loss can, however, be explained by the diabetes meds.
And yet. She will not consider this as the explanation. Correlation does not equal causation - the fact that she started taking an appetite suppressing, insulin balancing drug at the same time as her appetite got suppressed is a wild coincidence. Will no one be concerned about her weight loss? How about if she mentions it all the time? Then will we give a shit?
The combination of endless humble bragging ("Guess what size this cardigan is? You won't guess. It's a SMALL. Try it on. I bet it won't fit you - but try it on!") and performative lack of appetite is exhausting.
I am taking mounjaro for weight loss and I am, objectively, much slimmer than her now, and it is driving her MAD. Especially as I haven't told her, just because I cannot be arsed with the conversation it will trigger. My sister knows, my dad knows - we're just keeping it among ourselves, because any conversations about weight and my mother have always been a fucking nightmare. She's still trying to palm off clothes of hers that don't fit any more, and she still makes me try them on in front of her in the hope that she can humiliate me by evidencing I'm bigger than her/make herself feel better by evidencing she's smaller than me. I can't work out what drives her - but she is always trying to get me to try her fucking clothes on.
My husband spends a lot of his time trying to change the subject when we are all together. Same with his mum, actually - she is obsessed with my weight loss. I explained to my husband how her constant questions and wanging on about it made me feel a few months ago and he was initially defensive - like, his mum's just being nice, it's compliments - but now he's seen how rabidly both his mum and mine go on about weight and link it to appearance/moral strength, he can see that their generation is fully fucked up and is a superhero trying to deflect both of them when they go off on one about my weight/their appetite/food in general.