"Perhaps you remember that we have two girls. It is for the sake of the children mainly that we have to care for. Our own fate is of less importance."

"I know that it will be impossible for us all to leave even if most of the money is refundable, but Edith urges me to leave alone or with the children."

78 pages of newly revealed documents show how strenuously Otto Frank (Anne's father) tried to get his family out of the Nazi-occupied Netherlands and into the USA. His mother-in-law was still alive so he was also trying to get her out of the country, with his wife and children.
    The procedures for entry to the USA weren't standardized. It was very complex trying to find some clearance or convoluted procedural path out of the occupied country and into a neutral country. Despite having an American friend, Nathan Strauss, from their college days with strong 1941 connections to the US government, the arrangements just didn't come together. A series of administrative and logistical problems arose over time, no matter whom he turned to. Strauss, Mrs. Strauss, and Edith's brothers (Anne's uncles) made generous efforts, but it was not enough against the various bureaucracies.
    The letters span from 30 April 1941 until 2 February 1946. Not all 78 documents are from Otto; many are replies and documents. The Time article said the last letter Otto wrote before they went into hiding was on 11 Dec 1941. I imagine most of the documents were from before then, with just a few after the war looking to bring Otto to the USA before he decided to remain in Holland.
    Early in this effort, in the summer of 1941, Otto also started arranging a hiding place in case these plans failed. It was wise. In December, Otto apparently stopped seeking emigration, probably because the US had declared war on Germany and so was no longer neutral to the Netherlands (a German-occupied country). His mother-in-law died that winter and the family went into the hiding place in July. They were there until they were discovered and sent to concentration camps in 1944.
    The newly discovered papers had been in the YIVO Institute for Jewish Research in New York for years before a clerical error revealed their presence. The file was one of "tens of thousands" transferred from a refugee service(s) to YIVO. The Time article said that the files were transferred slowly to YIVO from 1948 until 1974. It may be they arrived before Anne Frank's name was famous. It may be that the file was never properly labeled for all those years: during the massive indexing effort, the Frank file was stumbled upon in 2005. While sorting out the resulting copyright issues, they kept the letters secret. The letters are being shown in YIVO in an exhibit until March 20th. See also:

• YIVO's very good article, explaining the situation the Frank's faced and the efforts of their friends in the USA on their behalf.
• Time's article
• The BBC site includes a slideshow of seven of the letters.
• The New York Times article includes pdfs of the first letter. They may seem to have problems (fuzzy and jumpy), but wait and then zoom in. They were very clear at 200% zoom for me. Please also remember English was not Otto's first language.