Wait until you're 12 weeks. You're chances of a miscarriage have dropped a lot by then, and it will have given your brother and his wife some time to settle in with their child too.
Really, if you want to, tell everyone now! They're still going to be excited for you. I actually found out I was pregnant on the day one of my friends had her baby... I didn't tell her right then, but the next day I messaged her on MySpace, thinking that she wouldn't check it for a few weeks. WRONG! Lol, she was still just as happy as could be for us though!!
My mom had been wanting grandbabies for a while... now my oldest sister is pregnant... due Sept. 7, I'm due November 3rd, my older sister is due December 31st, and my brother and his wife are expecting now too. I can tell you my parents were all very excited to hear they were having more grandkids!