Physical abilities are only part physical.... they are also part mental!!! Yes, training harder than her will help but since she is already faster than you chances are she has something else developed more. If she is a twin and you don't notice her as being more physically fit then the biggest difference is probably in her head. Try to create a mantra, something you repeat in your head over and over as a practice. Let this mantra be short and sweet and most importantly, believable! Try something like "I'm better than before", and when you say it imagine yourself doing just a little teeny weeny bit better everytime and because you believe it in addition to training, not because of training alone. Belief is more powerful than a week of training if you have a strong enough will. Whatever happens though, whoever's advice you take, believe you will someday be faster than her, believe it like it's true AND IT WILL BE!