Ah, In-Laws, can't live with 'em, can't strangle them and throw them in a ditch. What to do! First, some unsolicited advice. Either directly or subtly, don't allow anyone to 'self-invite', it's a sure path to relationship hell and loss of self respect. We have a cottage in the White Trash Mountains in PA to which fair weather realtives are constantly inviting themselves, as in your case, not to spend time with us, on our terms, but when we're not there. They also request we give them the key and we haven't and won't. We reply to these requests by giving a list of dates we'll be there and entertaining and invite them. None ever accept.
As for the advice you asked for, it's all been said. You'll need all your keys for EMH, they don't have to be informed you're not doing EMH. If you're one of those who refuse to blur the truth under any circumstances, go to the front desk, tell them exactly what Minnie the Moocher proposed and I'm sure the CM will shoot down the idea. Inform your dear friends of the reply. Good luck.
Bill From PA