I would like to begin by explaining that I do not have correspondence from many of the siblings. The best way I feel I can answer this question is to write the answers I have and where they come from. I will let each of you decide what you think.
On August 30, 1957, Allene Jensen, interviewed John and his sister Belle about the success of Papa Married a Mormon. In the second paragraph she writes
John Fitzgerald in collaboration with his sister, Belle Fitzgerald Empey, wrote the recent best seller, Papa Married a Mormon. Mrs. Empey says her name is not included on the book because of its being written in the first-person. An account of their family's life in pioneer Utah together with a good-natured lesson in religious tolerance, made it an overnight hit.