It's the kind of thing that's hard to understand if you don't have someone in your life doing this to you.
My PILs would often use the "we need a break" argument but they would often push it too far.
For example, they once went to visit BIL and SIL on a whim, stayed for a week, drove 100 miles or so out of their way to call in on us as they "were passing" on the way home, invited themselves to stay the night, stayed for four days. Then they borrowed money they never paid back so they could put petrol in the car to drive home. Within half an hour of them getting home, which was a four or five hour drive away, they rang us to tell us, not ask us, that they were coming back and would be staying for another week or so as MIL and other BIL had had an argument and she "need a break."
And the snooping can drive you mad as well.
Like the time they opened a private letter and read it "because it was just left on the table for anyone to see." In the sealed envelope, waiting for the person it was addressed to to get home and open it.
Or the time they let themselves into BIL and SILs house while they were on honeymoon and threw a party for their friends and neighbours, during which they opened the parcel the wedding photographer had delivered and showed everyone the wedding album and video before BIL and SIL had seen them.
I could go on an on with this. They would also go directly to BIL or DH and put them in a position where they felt unable to say no. And okay, they were talking to their sons, but they were clever enough to do it in a way that excluded their wives and it's that kind of sneakiness that gets to you.
If you know that someone is doing this to you all the time it does get to you. DH has admitted that he feels unable to say no to his mother because he's been told all his life that she will have a mental breakdown and it will be his fault. Not everyone's situation is that extreme, but I can understand why the OP feels the way she does with a MIL she knows will snoop and a DH who has left this to the last minute as a way of manipulating her.