Mikky, I am also sorry for what you had to go through. I think the question "Why didnt I?" is very common among survivors of any type of abuse. I'm a survivor of multiple types of abuse, and over time I have learned to change "Why didnt I?" in to "How could he?" or she or they. Putting the blame on yourself can make more situations triggering. At least that is what I have found in my past.
I'm so glad you are safe from your brother...hopefully that brings you some happiness.