How can my 11 year old sister lose weight? she weighs 58.6 kg and is 5 feet 2 inches.

Exercise: Regular exercise ( aerobic, muscle building, sports etc.), and healthy eating (low in carbohydrates, low fat, required protein, fresh fruits and vegetates). General principles are the same irrespective of age, gender unless someone has a specific health issue requiring special diet. Always talk to the doctor before starting a new sport activity and/ or modifying diet. I don't prefer diet pills.