I agree, tell her to hire a sitter. Or to bring the girls with her and S10. She could actually be making it a fun trip for them, if she wasn't so hell-bent on dumping on you. Seems like you may need to point out a variation of the prior quote about lack of planning and emergencies...
If I was one of the 17 or 19 year olds, I would be insulted that it was thought that I needed a sitter.
The 19 year old is a college student. The 17 year old is months away from being one.
Sounds like she doesn't realize that children/people rise to what is expected of them...
