SIL knows I hate people dropping in without at least a text message or call to let us know.
DH has told her this multiple times too and it did seem to have worked for a while.
She's just turned up. We're both busy with domestic chores, I'm upstairs cleaning and listening to podcasts, DH was outside hacking back shrubs.
Our kids are adults, we're empty nesters, for context, but SIL used to do this when they were at home, as did her mum, my late MIL.
DH just sheepishly came upstairs to tell me SIL was here. I've carried on hoovering - I 'm a complete mess, in my cleaning clothes and CBA to go down.
YANBU - stay where you are and don't reward her behaviour
YABU - be the bigger person, go down and be the perfect host through gritted teeth