When Cara, a reclusive writer, befriends a woman who believes she’s the fictional character Jane Eyre, she finds a new sense of purpose in helping Jane navigate the complexities of modern life. When the writer’s estranged brother turns up with questions about their past, Cara must finally untangle her own truth from fiction.

I like how you ended the logline using the words, truth, and fiction.

Here is an attempt to shorten your logline a bit. Hope I didn’t drop too many story elements in doing so.
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“When she befriends a woman claiming to be Charlotte Bronte’s fictional Jane Eyre, a reclusive writer must help Jane navigate the modern world all while trying to untangle the truth of Jane’s existence from the fiction.”
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Hope that helped