S.J.
Who is paying her bills? Who buys her food? Is she paying rent? My youngest is now 18.
In the beginning it was rather funny, I would tell her she needed to be home at a certain time ( for example) and she would say, but I'm 18 I don't have to.... My response made ME feel free. I said "Yes you are. Which means I am no longer responsible for you and your actions. If you refuse to follow my rules, when you do come home, all your things will be on the front porch. Your cell phone will be disconnected and you better not get sick, because there will be no insurance. And I hope you like taking the bus, since that will be your only mode of transportation."
After she picked her chin up from the floor, she decided it would be worth her while to continue to follow the rules.
She still gets a little mouthy from time to time. But a quick reminder is always followed by a respectful apology.
Don't put up with disrepect in your home. It sounds like your children might be following her lead. You really need to lay down the law and follow through. If it comes down to it kick her out. It might be the best thing for her.