Why is the book called Jacob Have I Loved?
You are referring to the book by Katherine Paterson. The book broadly speaking is about the sibling rivalry between Sara and Caroline. Caroline is by far the more popular of the two. She is prettier, more talented, and so gains attention. The book is about how Sara frees herself from this complex and finds freedom and peace.
The title of the book might sound odd, but it actually comes from one of the most famous sibling rivalries in the history of the world. It comes from the Old Testament. In the book of Genesis, we read of Isaac, one of the patriarchs. He has two sons (twins), Esau and Jacob. In this story, Isaac loved Jacob more than Esau; hence, the rivalry started.
The apostle Paul in the New Testament stated: "Jacob I have loved, be Esau I have hated" in Romans 9:13. Here Paul is referring to a different rivalry, but similar dynamics are in place.
In conclusion, the book by Katherine Paterson uses the rivalry in the Bible as the backdrop to her story.