[DubaiSis]

And something OneHeartOneWay said is very good advice: there is no need to overshare. Although I would advise them that you may be late, they may well be able to put two and two together, and I would NOT recommend lying to them, there is also no reason to put it right out there for them. If you are pressed, you can always say something like you really want to pledge their group, but you have to have a back up plan and unfortunately the other group's event is earlier. This is not a lie because I'm sure you really would like to pledge that chapter, or you wouldn't be preffing them in the first place. But again, I wouldn't say this unless you're feeling backed against a wall.