Deceased Brother's Widow's Marriage Act 1921

I have not yet read the novel, so maybe there is an explanation I miss in why the managed to get married only so many years later, after the end of WWII. It seemed to me they had to wait for the law to change. However, when I looked for the law in wikipedia:

1907 act and subsequent legislation[edit]
The Deceased Wife's Sister's Marriage Act 1907 removed the prohibition (although it allowed individual clergy, if they chose, to refuse to conduct marriages which would previously have been prohibited), but the Act did exactly what it said and no more, so, for example, it was not until 1921 that the Deceased Brother's Widow's Marriage Act 1921 was passed. The Marriage (Prohibited Degrees) Relationship Act 1931 extended the operation of the 1907 Act to allow the marriages of nieces and nephews by marriage as well.

The Deceased Brother's Widow's Marriage Act (Northern Ireland) 1924 was passed to remove doubts as to the application of the Deceased Brother's Widow's Marriage Act, 1921, to Northern Ireland.