Is Valentine a key to Ender's success?

I read through chapter 10. So far, Ender has been able to complete tasks and games thought to be impossible. I feel like, however, he is always picturing his sister. For instance, when he kept seeing Peter in that one game. Ender was never able to pass it until Valentine wrote him a letter. Although he knew the letter was somewhat forced by Graff, it still did something so that when in the game, Ender could go through. I just wonder if she has been his motivation (almost) to succeed.