If you don't feel like now is the time to tell them, then fine, give them some reason you can't partake (I like the medication one), and get through the weekend. I was in a similar situation and just figured "Well, if anyone asks I'm just going to be straight up with them, it's time to stop hiding this." Weird thing was NO ONE asked! I think the most important thing is to be ready for it and do what you feel you have to do to not drink.
"Everyone relapses..." be careful with that one. No one WANTS to, trust me, it's no fun feeling like you messed up and let yourself down and now you have to start from the beginning again.
You can do it. Good luck.