help!

Mrs. B
So im scheduled to go to Vegas for my youngest sisters birthday next week from Tues-Thurs- AF should show on Friday/Saturday. We're on cycle 4 of TTC#1, and even though I think we didn't BD enough around my positive OPK, there's still a chance. 
I don't want to drink to find out later that we're pregnant...but they're my sisters and will suspect something if I'm not drinking, which will make the BFN even more emotional and worse. My husband and I are trying to keep this private since it's our first etc. 
Ideas/advice please??