Our code word/bat signal is anyone of us asking what time it is or bemoaning what time it is while looking at a clock or watch. (I only wear a watch around my mil;).
I've spent years letting her walk all over me in our home, but stopped last year after fil died. It feels so good, but freaky/scary saying no. Bullies have to get a solid no with no neg. following as that is how they work their way. I finally realized mil, would never in a million years allow me or anyone else to get away with what she does. That was my ah-ha moment.
I pray for her, I love and honor her, but she doesn't get to rule me or things in my home.
Imo...fwiw...I'd make that sil a hotel res. and even pay for it not to put up with her. (If she didn't like my hotel choice, she could stay at another.)