HELP!

I need some advice from my fellow mommies-to-be!
We are leaving for a
WEEK LONG family vacation on Saturday with my husband's side of the family. His sister and her family and his parents will all be there along with us. I wasn't planning to tell anyone about the baby until we have our first ultrasound on 8/29.
I know lots of wine and margaritas will be involved at this reunion and I'm worried I won't be able to avoid it without notice. I can just see my SIL blurting out in front of everyone "ARE YOU PREGNANT or something???" To which I will be unable to hold a straight face to respond with "what are you talking about???" And the beans will be spilled about our little bean, lol!!
I know there are worse things that could happen, but I have special plans for telling my kids about the baby, so I don't want them to find out this way.
What would you do?
Tell SIL (to keep it secret) and no one else?
Tell SIL *and* MIL... so MIL doesn't get her feelings hurt?
Or try to make it through the week without anyone noticing (SO HARD!!)?