Hi Surround/ alone,
Welcome to the forums and thanks for being here. I'm really sorry for all your losses.
To answer your question though, yes - it's normal to feel guilty for being alive. Sometimes this is called survivors guilt. Without knowing you or your brothers, I am already positive that there is nothing that could have been done to prevent their deaths, however I can understand how that feeling can be so incredibly strong.
Often, people have these constant 'what ifs' - what if I'd done something differently, or been somewhere else, did/didn't say/do that. The questions are endless and it's so frustrating having them unanswered. Given that we have no answers, it's the next best reaction to blame ourselves. You're absolutely not alone in this.
I hope this answers your question. Feel free to let us know how we can support you more and I hope you enjoy being apart of these forums.