
Stephanie C. answered 06/20/19
B.A. in English, University of Arizona
This looks to me like a biography written from interviews with a living (at the time) subject. Since you used the interviews as primary source material, supplemented with historical research materials as secondary sources, you are the author. If you were publishing this professionally, the authorship credit would probably be part of legal negotiations in your contract. If you want to grant the main authorship to your MIL for personal reasons, a co-authorship makes sense. TITLE by MIL NAME with YOUR NAME. The "with" acknowledges that the life story comes from her and the writing comes from you.
It sounds like giving proper credit to everyone who participated in the process is important to your whole family and certainly no one should be left out. This is why books have acknowledgement sections. Professional translators and transcribers are definitely given proper credit, as are research assistants and anyone else who helped the person with their name on the cover as "author".
In your case, it would also make sense to add an introduction detailing the creation of your book, just as you described above. No need to limit yourself to one word to explain each person's contribution. How your whole family collaborated on the project is an integral part of the larger story of your family history. What better way to bring your MIL's story into the present than to tell readers about your generation's work to keep the history alive?