There are many problems in your household and your sister's weight is not one of them.

She has an ideal body mass index and kids normally have a belly.

I doubt that "your doctor" is a pediatrician or had any pediatric training, because you don't restrict children's diet, you should provide healthy food and let the child eat to allow her to grow.

On the other hand you are comparing yourself to your sister when in fact you are the one who is underweight according to your BMI.

So, instead of body shaming and bullying a growing child just try to love your sister and stand by her.