Maybe it's just me and I should have finished the book, but Lucinda Riley's The Seven Sisters does this in the worst possible way imho.

I was hoping about some good stories about those seven interesting sisters finding their heritage, but instead, the story just totally switches to the ancestor and the whole book is about the ancestor, not about the girl I thought the story was about.

I would have probably liked it, if the girl in the present found little snippets about the girl in the past, but the whole book is just a wall of text about the girl in the past...