The presumed unattested ancestor of these languages (more properly called Proto-West Germanic).
1940, Charles Hart Handschin, Modern-Language Teaching, World Book Company, page 165:
German and English are sister languages, descended from West Germanic.
p.1993,a.2002, in Donald McLean Lamberton (editor), The Economics of Language, Edward Elgar Publishing (2002), ISBN 9781840648027, page 93:
The next logical step is to note that the Scandinavian languages, descended from a North Germanic parent, are more distant historically from English than are Dutch and German, which, like English, are descended from West Germanic.
2010, Peter Trudgill, Investigations in Sociohistorical Linguistics: Stories of Colonisation and Contact, Cambridge University Press, ISBN 978-0-521-13293-0, page 50:
The degree of actual Dutch influence on the Norwich dialect is somewhat difficult to determine for the rather obvious reason that Dutch and English are closely related languages, both descended from West Germanic, and resemblances bewteen Dutch and forms of English are therefore most usually due not to the influence of Dutch on English, or vice versa, but to their common origin.