You definatly don't need to lose any weight then. You are the same height as my best friend who weighs 118lbs and she is really slim, me and all my friends are really jealous of her figure! Your little cousin could try losing weight in a fun way. She could take up a new sport or just muck around playing basketball or tennis or something with her friends or cousins. Encourage her to be active. She could also help do the cooking at home, where she has to plan a healthy meal with the help of her mum/dad and then help them to cook it. This will help her to learn how to eat healthily.