I've written here several times about my SIL. Right after we got married, she developed a habit of having "seizures" and "fainting fits" at church, at stores, at the movie theater. Ambulances were called. ER doctors examined her. But there was never any legitimate reason for her to faint. And she didn't have a seizure disorder.
My husband was working night shifts and I lived 20 miles closer to the hospital than my inlaws. I kept getting calls from SIL saying, "They're rushing me to the ER, please come down, I don't want to be alone!"
And I went. Three times. Each time, SIL perked up the moment I got there, asking for Sprite from the convenience store down the street because the hospital only sold 7up, turning on her favorite TV show so we could watch it together. So I said enough. There was no reason for me to be there. There was no reason for SIL to be there. She wasn't sick. And my hubby wasn't thrilled with my driving into the bad part of town at night, alone, to get to the hospital.
On the third time, I told SIL, "This is the last time I can do this. This is the third time I've come to the ER in two weeks. I can't keep doing this. You need to make other arrangements the next time you go to the hospital"
The next week, she called again, having "fainted" at the grocery. She wanted me to come to the hospital, and could I bring her some fries from Wendy's? I said, no, I'd told her that wouldn't be possible anymore and she needed to make other arrangements. And then I hung up.
The fainting spells mysteriously ground to a halt.
You just have to say no. Don't make excuses or give reasons because she'll argue around them. Just say no.