If at all possible, the LW and BF should pay for a room (or two) at the hotel where the in-laws are staying and tell the sibs "Sorry, we couldn't possibly be good hosts right now so here's the hotel". Repeat as necessary. Give no specific details about why right now is not a good time; specifics allow people to argue with you. I promise that this will be money well spent.
Secondly, find something for them to do that you are OK with. Maybe have them put together a scrapbook or decorate the front yard; something that you are OK with letting them do as they want without oversight (maybe the front yard isn't such a good idea, come to think of it). Tell them how thrilled you are that they can do it because otherwise it wouldn't be done. Keep them busy with something that you don't feel you need to control (and I don't mean control in a derogatory way, you have the right to arrange this ceremony as you see fit).
When the sibs want to make suggestions that you don't want to take, instead of saying "Hell NO!" or whatever variant you're considering, smile and say "That's something to think about" or "I'll give that some consideration". You are not obligating yourself to do anything but the person making suggestions feels that he or she has been taken seriously.
You're going to be stuck with these people for a long time, so you may as well start getting creative about dealing with them. And save a bottle of champagne to drink after they leave.
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Salon headlines in your mailbox