My sister is kind of aneroxic and im worried for her. she's restricting herself to less than 1000 calories?

she restricting herself to 1000 calories and below a day and nothing above that. she's my twin sister. and we're only 14 this year. she's severly underweight, at a weight of 94lbs at the height of 5'3. im 103lbs at the height of 5'3 too. i know that im also underweight but i used to be 104 lbs. it is really pressuring as when people compare me to my sister, they call me fat and big. i really dont know what to do, Someone please give me advice! no matter how hard i try to talk to my sister, she will nvr ever listen to me.