I can access Family Library and share books with me wife. They are marked as shared on my account but she cannot access them on her mac or iPad. There is not a setting for Family Library on her account. Although books she has shared with me appear in my Family Library.
she needs to turn on Show Family Library to see them on her Your Content and Devices page on the Amazon website. You did check Share All didn't you? See images for that page:
If she is part of your Family Library she should be able to see the Households and Family Library under her Your Content and Devices page. She can reach that on a computer on the Amazon website under the upper right menu