I will have you know that I have now gone FOUR week long visits with my mother, the remote controller, without having a single cross word with her. I think if I can do that, you can certainly behave with your MIL for a few days.
Of course, I made merciless fun of her and had several belly laughs at her expense, but I didn't get mad or even irritable. She was pretty pissed, but I was calm and serene.
I do not like her doing my laundry, not because I'm worried about her seeing my undiepants but because she doesn't sort. My Dad has more pink t-shirts than anyone you've ever seen. So when she told me to go get my things so she could wash them, I just told her no, I'll do them when I get home. She insisted, I said no, I'm ticky about my laundry. She started getting bent, I asked her if she was seriously going to have a fit because I wouldn't let her wash my clothes. She proceeded to have a fit and I said, well, I guess that answers my question.
My dad was howling
At one point she wanted to drive my rental car for whatever reason. I told her no, it's a rental and she wasn't on the insurance. This was right after she told me her car was in the shop because - ready? - she'd driven it into a concrete wall.
Really.
She huffed and said, "You act like I'm retarded." I replied, "No, YOU act like you're retarded." This made my dad and I both crack up laughing, and she pouted until we'd recovered, then all was well.
But I did not get cross or feel frustrated in any way. And I think if I can deal with that crazy person, you can deal with someone who's actually trying to be nice. Just tell her you're ticky about your laundry - make it your problem and not hers.