hey, i've struggled w disordered eating (i dont want to say eating disorder for fear of self diagnosing but honestly atp) for nearly 2 years (ofc this is nowhere near as long as a lot of ppl and my heart goes out to them), ive got a lot of close friends and family with various eating disorders and eating problems, and ive done a lot of research. you havent included much detail in your post except that shes been eating significantly less and being aggressive about it, and while that's def a warning sign its not enough to be certain/confront her about it. here are other possible indicators:
look at her behaviour while she eats. does she try to distract you from seeing the amount she puts/leaves on her plate (by talking a lot, or arranging her food so it looks like more/smearing leftovers around her plate so she can leave more food)? does she eat abnormally slowly/cut food into small pieces/get irritable, nervous, or otherwise uncomfortable while eating?
has she been talking of dieting at all? or researching dieting? a good way to check the latter would be (not to go through her search history pls theres nothing worse even if youre desperate) to say anything factually incorrect about weight loss in front of her - she'll almost definitely correct you if she knows more than she "should".
outside of food, something ive seen really commonly is chewing gum all the time/drinking excessive amounts of water/having diet drinks often. gum keeps your mouth occupied, water and diet drinks keep you full. theyre all classic disordered things. have you noticed any negative self-talk on her part? on any subject - eating disorders dont only ruin your relationship with your body but also make you dislike almost every aspect of yourself.
an important one: has anyone in your family struggled with eating disorders, or even any addiction? its a repeating pattern in people like me - mothers in particular displaying unhealthy examples around food and ingraining them into their children, on top of the genetic factor. almost everyone i know with an eating disorder has at least one family member who had/has one too (i have, what, 5 family members with eds that i know of? including my sister and mother).
if she has a history of poor mental health AND body image, i'd be particularly wary.
if she does have an eating disorder, it's better to talk to her about it asap - she'll deny it and get angry but these disorders are sneaky and they take root fast. once the damage is done, its done - only 20% recover fully, 60% are in and out of recovery their whole lives, and the remaining 20% die. i can't stress enough the importance of quick action, especially if shes taking up starving herself as a coping mechanism. with my sister, our parents (my family mostly doesnt know about my eating habits) noticed far too late, made her gain weight by threatening her with inpatient, and essentially did not fix her problem at all (id go so far as to say they made it worse). i'd say the best thing you can do is sincerely try to understand the struggle, i think the reason my sister kept confiding in me through it all is that i never judged her (for reasons that she'll hopefully never know) and tried to understand even where our experiences diverged completely.
eating disorder sufferers are incredibly hard to help - mostly because a lot of them dont want help. they think they "need" their disease because it will make them thin or help them cope. watching a family member or friend destroy themselves like that triggers a reaction in us that is protective and ofc terrified, while they trivialise their issue and continue feeding their disorder (pun fully intended, i cope with humour lmao). its painful, especially when you realise how powerless you are - bc eating disorders are mental disorders and even when a person seems recovered, the thoughts are often still there ("your thighs are fat" "skip lunch today" "you dont deserve to eat") and relapse usually comes fast.
overall, eating disorders are horrible and an extremely sensitive topic for both sufferers and their loved ones - you'd better hope (and i sure do) that your sister doesnt have one, because its hell. please take good care of her as far as youre able.