I think this is so hard!!! Especially because this is a whole trip away for you! If you were at home, having a traditional wedding, it wouldn't be such an issue, but because this is being away and you get to celebrate your wedding for a few days (which is why I am so excited about my DW!!!!) it makes it harder. Maybe your FI can just sit hiim down and talk to him about it (I mean...he is the BM, so they should be pretty close?) and explain how much this trips means to you as you prepare to celebrate for your special day, maybe he will completely understand? I don't think it is harsh to ask him to wait until the day after
Haha..have him tell his BM that she will be so much more hyped up after watching the two of YOU get married, that the morning after, at sunrise, she could just be blown away about how now SHE gets to plan hers!! hope that help!?! )