What can you do to stop what people call "emotional overeating"?

Counseling. Truth be told, overeating sometimes hides a primary psychological issue, such as depression. You may want to encourage your sister to seek counseling to elucidate the cause of her overeating. If she's just gluttonous, then emphatically point out the error in her ways and provide her with support and encouragement to change.
Overeating. Will power, determination, set a goal and work toward achieving, seek social support, psychotherapy.