Hi there, and welcome to PC.
No...whatever happened is absolutely NOT your fault...not by any means at all. I am assuming your brother was older?
Understand, you're looking back at events that took place when you were 11 (with no true comprehension of sexuality) with the knowledge and understanding of an adult woman. You were unable then to make the full range of rational decisions that you are now. You cannot, whatsoever, be held accountable for what happened. Please, don't let that haunt you.
I hope I was of some help...if you need me, please feel free to PM me. I'm an open ear.
Hugs, and all my best,
Harley