What are the problems using BMI to calculate obesity?

The BMI does not take into account body composition at all with its measurement. For example, a person with is 5 feet tall and weighs 110 pounds has a BMI of around 21. -- These are the measurements for my 8-year old daughter. Now, I also have a sister-in-law who is 5 feet 1 inch tall, she is a serious bodybuilder, and she weighs 125 pounds, making her BMI over 23. My sister-in-law is DEFINITELY NOT "fatter" than my 8 year old girl, yet her BMI is higher than my daughter.