I diagree that Hazel is making decisions for Raymond and Gretchen.  Hazel/Squeaky realizes that there are more things in life than winning.  She knows that she could put her mind to anything she wants and accomplish it.  She has lots of medals and trophies to be proud of and display, but what she realizes is that Raymond doesn't have any of those things because no one feels he can accomplish anything due to his mental disability.  As Hazel starts to really watch her brother, she realizes that he has been with her every step of the way in her races; warming up, breathing exercises, and actually running the track.  Hazel sees that her brother has a great deal of potential to be a track star and knowing that Gretchen is very serious about track, she wonders if she would like to help her coach him.  Hazel wants her brother to have something he can be proud of and so that he can show others that even though you have a handicap you can still be successful.