On most threads where someone, anyone, is coming at short or no notice for whatever reason the OP of the thread is usually told countless times "Well he invited them, let him cook/clean/shop for their visit."
Which is exactly what the OP suggested to her DH, his parents asked to come, he agreed, didn't tell her, knows their standards and habits when in their house and now she's been called all sorts of names for suggesting to him what countless people on here recommend in situations exactly like this.
The OP doesn't have to be a filthy slattern living in a hovel or have 'secrets' that she wouldn't want anyone else to know about to object to this visit. It is possible to live in a reasonably clean and tidy home and feel that your pay slips and pile cream are private things you don't want your relatives poking through without being a terrible person and a potential nightmare MIL yourself.
peacefuloptimist I find your post a bit depressing too, to be honest. You write like you feel the parents own a child, even an adult child, and have a claim to the home they have made with their partner and children. I gave birth to my son, DH and I are raising him, we are investing everything in him because he is our only child but never, ever in the future will I believe I have more rights than his wife/partner in their home.
You say you were going through your sisters things when you stayed in her home and then put it down to her children emptying cupboards and drawers. If you were just picking things up and putting them away again then no, that's not snooping. If you were taking things out of envelopes or reading through diaries as you picked them up, then yes that is snooping.
There's nothing wrong with sharing the private details of your life with your family if you choose to do so but if they are discovering the private details of your life by using their time in your empty home to look in drawers they have no need to open or read your mail or look at your filed bank statements then that is wrong of them.
We often leave bills, appointment letters and things that we are still dealing with clipped to pegs hanging on the back of the kitchen door. They are there for our reference and if someone sees them there hanging there, well, fair enough if they catch a glimpse of a price or appointment time etc, because we left it on view. Complaining about that would be as pointless as complaining that they saw an ornament on the mantelpiece or a framed photograph on the shelves.
But if someone takes a bill or appointment down from the peg and reads it, no, that's not on, that's very different because they know it's none of their business but they've gone beyond just noticing something to actually removing it and reading it anyway. It's gone passed noticing something to snooping at something.
There is a balance and obviously it's nothing secret if it's hanging on the back of the kitchen door. But it's still obviously something that's ours, not theirs, and nobody else coming into the house has any right or need to take it down and read it in detail just because we haven't locked it away. They ought to know it's not their business just by the fact that it's not their house, it's not addressed to them, it doesn't belong to them etc.
And it's even worse if they are entering rooms they don't need to go in or looking through drawers and cupboards they don't need to look in.
Just because some people wouldn't have a problem with that, it doesn't mean that someone who does has something to hide or is a controlling freak trying to manipulate their entire extended family. It's just as fine to feel that you have a right to privacy in your own home from friends and relatives even if you have no experience of one who snoops as it is to feel your house and life are open books to anyone who wants to come and look at them.
Locking something away actually wouldn't stop my PILs or some of the others being discussed here. If they found a locked drawer they would search for a key. It can be hard to understand why that might be a problem if you have no experience of living with a relative who would do that. Or if you are the relative who would do that.