http://takingnote.blogs.nytimes.com/2015/11/17/who-wrote-the-diary-of-anne-frank/
Robert Faurisson discussed this back in the early 1980's -
http://codoh.com/library/document/2016/

By admitting that the diary was co written by Otto Frank in order to keep the rights the Anne Frank foundation is showing that its major driving force is making money not historical accuracy -
https://furtherglory.wordpress.com/2015/11/25/anne-franks-family-in-amsterdam-was-denied-entry-into-the-usa/

Its always been suspected. For example the lack of entries discussing fights between the parents always seemed odd.