This is an important concept. Body image is how we see ourselves and how we perceive others see us. Good body image starts by acceptance. We grow up to be who we are largely based on genetics--what the family looks like. We can change weight, tone, and condition by good diet and exercise e.g "taking care of yourself". But don't obsess over it. tall people will be tall and short will be short. Some younger siblings will be bigger and stronger than older siblings. It is what it is. Reinforce to your sister what matters to you in your relationship and that it is not about the body.
Wayne Yankus, MD, FAAP
expert panelist: pediatrics
Body image is a person's perception of the way their own body appears. If you had "poor body image," for example, you might think of yourself as overweight or unattractive. This can have serious consequences, as poor body image can lead to eating disorders and depression. Strategies to improve your body image can include anything from a healthy regimen of diet and exercise to consulting with a therapist or counselor. Often, adolescents struggle with poor body image as they get used to their developing bodies, and this will subside as they grow and mature into themselves.