I find out my little sister has it

My little sister has been weird with food the past year and today we forced her to stand on the scale and I couldn’t believe my eyes. She was 37 kgs. Just a year ago was 64 kgs. Me and mum started to cry and so did she. She’s extremely thin sleeps all day, she doesn’t smile or even talk to anyone. I’m so heartbroken to see her this way. We did try to help her earlier too but she would throw a tantrum and cry.

After talking to her all day she finally agreed to bring her BMI to a normal range ‘20’.! Currently her bmi is 14 . Today she had a lot more than usual but What steps should I take in meal plan. ?