How can I keep it a secret now?!

Oh lord ladies. .. listen to this one. So we are thrilled! We got our BFP and I went to the doc Fri and it's confirmed. ♡ My husband and I agreed to wait 10 -12 weeks at least to spill the beans because I'm older and ...well....cautious. But NOW 2 days ago my brother called and said he is deploying for 6 mos and leaving right after Christmas! AND then my sister in law is getting married Jan 17, 2015. Of course we don't want to steal her thunder and announce the baby after the wedding. BUT YESTERDAY she called and asked me to be an emergency bridesmaid 2015 because one can't make it! Omg....should I tell them or wait. What do you think?? Yikes... thank youuuuu