she's probably the mastermind....
I know you're probably kidding, but she probably at minimum knows some people that may come in and out of the house for "tea" or whatever.
With that said, family dynamics can be very different in different households. There are two wide spectrums possible here, and both are not that far fetched in my mind... 1. She knew they were planning this, or 2. There was a section of the house she never went in and was oblivious to it all.