Underweight child?

Just wondering if my sister is a healthy weight. She’s 6 and tall for her age. She literally is tiny and very pale and you can see all her ribs and her spine. I weighed her on my scales and it said she weight just over 2 stone so like 29lbs. I don’t live with her but whenever I see her I’m always giving her food and stuff but she does tend to leave a lot of food and likes to eat more sweets and chocolate so maybe she just has a small appetite. Just wondering if this is a healthy weight or not