My reply would be: "Your problems are not my problems. Oh, by the way, I'm no longer offering weekend care at all." But that's just me
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Seriously - I would just have a frank, face-to-face talk with her and explain in no uncertain terms what you will and won't do, and how you expect her to behave. If she can't deal with it after that without all the drama, then I would probably terminate care. (That's coming from someone who rarely has to terminate anyone, BTW, so I do not say that lightly.)