Yea I agree with MamaAnna and srprisebean3 that it might be out of her control. Maybe bring it up by asking her if her parents require her to watch her sisters after school. I do think 5 under 7 might be a bit much for most teenagers, but it really depends on the 18 year old.
As for the money thing, I guess you have to decide if you want to pay her to watch ONLY your kids. Because unless you are paying her extra when she brings her sisters, you're not paying her to watch her sisters in the first place. You're paying her to watch your kids and the sisters simply divide her attention more. So you have to decide if you're paying for her presence or her undivided attention.
The mess thing, though, is unacceptable, a) because it's rude as a guest and b) because part of keeping kids in line is getting them to clean up when they're done playing. Say.. 10 minutes before Mom gets home.