Update: Bratty Breaks Her Silence...
So at the start of the holiday weekend Bratty texted SO to ask to stay 'one night sometime in July'. We had previously set rules in therapy last year for Bratty's visits (reasonable notice, specific dates, and adhere to the schedule). SO texted her back and let her know per our agreement in therapy we needed specific dates.
Bratty went silent all weeked as she does when she doesn't get her way.
This morning she finally responded to SO's text and said she still wasn't sure about dates and was looking into alternative stay options. YES!
At least she's now fully aware that we don't intend to simply ignore or forget about the boundaries we set last year. And as a soon-to-be 24 year old adult, they are free to stay elsewhere if they don't like the rules.
I'm pretty happy with how this turned out so far. I'm sure SO would like Bratty to be able to stay but there is way too much drama when she does (which is why we needed the boundaries in the first place).