Scootaloo really looks up to Rainbow Dash, and thinks of her as her big sister. However, Scootaloo doesn't have the number of feats that Rainbow has done, and once she ruins one of Rainbow dash's Daring Do books, she feels like maybe she doesn't deserve to look at Rainbow as her older sibling.

She gets some valuable advice from Eric Reed, someone that once felt that he didn't deserve to look at Twilight Sparkle as his big sister, with the advice that he himself once got from Starlight Glimmer, who he also thinks of as a big sister.

This story takes place after the events of my "A Giant Adventure to Equestria" story, and this story has some spoilers from it.

Here's the link to that story, if you want to read it first, or just curious.
http://www.fimfiction.net./story/427481/a-giant-adventure-to-equestria