Describe what Miranda did at camp that made her self-conscious to call Olivia?

One day, Miranda told people that she has a little brother who is deformed, because in the heat of the moment it seemed like an interesting thing to say. Miranda knew that spreading such a falsehood is wrong, but she has known Auggie for so much of his life that she feels entitled to this lie.

Miranda continued telling these lies and grew more popular; eventually, when she went home, she decided to reconnect with Ella but to leave out Via, since Ella would be an easier friend to have and would not ask her a lot of questions about the summer that she would not want to answer.