What can i do if my sister is 12 years old and weighs 160 pounds. is she over-weight?

Depends: The standards for weight are gender and height dependent.If the girl is 66" tall, which is where the top 3 % of 12 yo are, the weight would be considered rather normal.If she is average height (59") or shorter, she would be rather overweight .You can download a growth chart from the cdc site and follow this issue.See: www.Cdc.Gov/growthcharts.