Depending on the size of your current family tree, I would do one of these things:
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Export your GEDCOM file and import it as a new tree, delete everyone not in her family, and share it with her. Or just share it with her and let her delete people.
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If there are a lot of people involved, there are programs that will let you split a family tree. If you search for split GEDCOM file, you'll find a few.