Sorry you have to be dealing with this, it's probably adding a lot of stress you don't need.
If you weigh your discomfort with letting her in for 5+ hours to "help" while you resent her intrusion against your more temporary discomfort of not letting her in when she shows up unannounced, I'm guessing not letting her in is the more peaceful strategy. Personally I can imagine seething quietly just because another person is occupying my space, no matter what she's doing. Let's say you had a regular housekeeper, someone where there isn't a major emotional involvement, little chit-chat, and definitely not trying to 'baby' you, you probably still wouldn't want that person around while you're feeling tired and out-of-sorts.
Since her help is not helpful, I think JenJay's and Mr Tango's approach is probably best, prefaced by one more conversation with her where you and/or DF state CLEARLY that she must call ahead and ASK, not tell, about coming to visit. Then never let her in unannounced, and when she learns to call ahead, only accept those offers of help when you feel you could really use the help.
Some other options to deflect future questions:
"You know, I really don't feel like talking about it right now"
"Oh, I'm so tired of discussing symptoms, let's talk about something else"
"How about something more fun for everyone?"