I go thru something similar EVERY time my one sister and her brood, or my aunt and her brood come visit. Here are what I do:
#1. Try to plan stuff to do OUTSIDE THE HOUSE!!
Cast away cafe - call them and ask them if they still have the free admitance before 9am... Get up, get food, go and stay ALL DAY. lol Out of the house, tire the kids out, you can sit and enjoy your friend.
McD's in howell or brighton, lunch /snack and hours of play.
If the weathers nice, local parks.
#2. Your house, Your rules. Tell them. If you see them doing something, don't wait for the mom to step in. You obviously have different parenting styles. Just say, at MY house thats only for grown ups. Even if mom is standing right there... It may take a few times but same MO for them as for your own kids. Other wise you send mixed messages to your kids about how things are different and all bets are off when visitors are there.
Enlist your kids. Tell them (If they are old enough to understand)that they don't know the house rules. So if they see/hear something that shouldn't be happening to tell them WE don't do that in OUR house. If they don't listen, then an adult should be consulted.
I agree with the writing down a general list of house rules. Put it up BEFORE they get there so it doesn't look like its just for them. Sorta run over it (not literaly) a few times with your kids so they know what it says. Things like no hitting... And a punishment system. (timeout for ... min per year, or having to sit out till the game is done, leaving the room etc.)Then if something happens all you have to say is #4 says...
Set boundries. Set a play room. Only "approved" toys are in there and if something not in the room gets broken then don't gloss over it. Have them apologize and ask them what they will do to "make it up "to the injured party. They play IN THAT ROOM ONLY!!! (For us in nice weather its the front enclosed porch, in our other house a part of the basement was "roped off")
Set a 15 minute block of time BEFORE bedtime as a manditory clean up time. EVERYONE helps. Make it fun. Go to a dollar store and get a grab bag of assorted goodies (rings, glow sticks, etc). After clean up whoever cleaned (your kids included )gets to reach in and get a suprize. They'll probibly want to sleep with them so nothing sharp.
I know its stressful and yet exciting looking forward to the visit. But you are laying a ground work for easy future visits when you have 4 kiddos in your house...
My nieces and nephews LOVE coming to my house. Even with the rules that they arn't held to at their home. And every time they do, they know the rules are EXPECTED of them just as they are of my kids. Makes them feel like a part of the family and included when you don't make exceptions on rules and punishment.
side note: If they were adults and they lit a cigarette in your nonsmoking home what would you do? Let them sit there or ask them to go out till they were done? See, house rules apply to everyone be they 70 or 7.
Congrats on the Babys!!! And after the kiddos are in bed make some tea and relax.