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The real-life case that it's based on may be that of Bridget Cleary - she died in 1895 rather than in the 1900s, but perhaps it's close enough. There have been a few books about the case - two which I can find are The Burning of Bridget Cleary by Angela Bourke and The Cooper's Wife is Missing by Joan Hoff. I can't guarantee that either of those is right, though.
Thanks so much! I'll have to check with my mom to see if that's who she was thinking of, but I'll let you know if you're right. I really appreciate it. ^^
Finally got around to asking my mom, and it is indeed "The Burning of Bridget Cleary." Thanks so much!
Trying to find a book for my mom. She said she read it while she had the flu, so the details might be a little fuzzy, but essentially, she thinks it took place in 1900s, Ireland, and depicted real events. It was about a young girl who was accused of being a witch by her family, and one of the things they did to "test" whether she was a witch involved throwing her into the fireplace, and if she burned, she was a witch. Of course, she burned, but they let her leave the fireplace, and she started having convulsions from being horribly burned, and the family claimed this was more proof that she was a witch. She emphasized that this was extra disturbing because, again, it depicted real events.