Anne Frank addressed her diary entries to Kitty and there has been much conjecture whether this name referred to a real person or was inspired by one. The English Wikipedia article about The Diary of Young Girl contains the following sequence of unsourced statements about the identity of Kitty:
Theodor Holman wrote in reply to Sietse van der Hoek that the diary entry for 28 September 1942 proved conclusively the character's fictional origin. Jacqueline van Maarsen agreed, but Otto Frank assumed his daughter had her real acquaintance in mind when she wrote to someone of the same name.
The German Wikipedia article about Anne Frank's diary also discusses Kitty's identity but omits the unsourced statements. The Dutch Wikipedia article does not attempt to summarise the discussion about the identity of Kitty.
My question now is the following: what are the sources on which the claims by Theodor Holman, Jacqueline van Maarsen and Otto Frank are presumably based?