Realizing that I am a lady who believes in being a submissive wife (that does NOT mean being a doormat, btw), and I whole-heartedly believe the husband is the head of the household and family, here's my opinion(s).
1) Tell them, along with your husband, point blank. Seriously, do you think they would take kindly to your showing up unannounced all the time? Maybe, but probably not.
2) Sit your husband down and talk to him, just him. Tell him, again point blank, that this is very difficult on you. It's not that you don't want them around, but they need to let you know when they are coming...at least a day in advance would be nice. If for no other reason to be sure plans don't need to be changed and the house can be picked up a bit (I totally understand this one!)
3) When they show up and you have plans, just say "Oh, hi! Nice to see you. Okay, I've gotta' leave. Have a great day!" Go do your plans, even if it means leaving your husband at home with his family!
These are bold moves, but sometimes it's the bold stuff that makes others take notice that their behavior is unacceptable. Not being nasty with it, just point blank that it puts you in a bind and is rude on their part.
I wish you luck!!