I don't understand why you are complaining rather than just sorting the situation out.
Ring the hotel and say to them that you want to be in a room very very far away from Mr and Mrs X, as they are your in laws and you would prefer some privacy on your wedding day. Also ask if they can leave a note on the system which explicitly states that your room number isn't to be given out to ANYBODY - including family member - as no-one in your family has any understanding of the concept of privacy.
This will not be the first time they've had requests like this, and unless it's a shitty motel or something, then they will have no problems accomodating your request.
Tell your DP that you want to get ready together, without any other family around. If that means lying about your room number, so be it. If it means turning around and telling them to fuck off and stop following him, then so be it. If it means getting out the lift one floor too many and walking down to get to your actual floor, then so be it.
If your in laws start bitching about why you are being so secretive, then be blunt
"Well, we specifically wanted to be in a different hotel to other so we would have privacy to get ready together. You weren't considered some special exception to that so I don't see why you are acting all surprised about it now".
Seriously. One phone call to the hotel and a conversation with your DP. That's literally all this takes to resolve.