Donald's nephews first appeared in a Sunday strip in the US newspapers on Oct. 17th, 1937. They were created by Al Taliaferro, Disney's Sunday strips artist of the time. By the time the first strips with the nephews were reprinted outside the US, they had already returned to their home after staying on with their uncle for six consecutive Sundays in the US newspapers (*). In the UK, Donald's nephews were first heralded at the bottom rim (see above scan) of the back cover of the no. 98 (dated Dec. 18th, 1937) of Mickey Mouse Weekly. The art on this slim herald, by an unknown British illustrator, is rather substandart, but it is somewhat historically important for being the first non-American rendition of the nephews. Nevertheless, talented British artist Wilfred Haughton used them in his cover illustration for MMW no. 100; below scan is of the portion of the said cover with the nephews
...and below image is of the full cover:
And here is the first strip with the nephews as reprinted in the color central pages of that issue:The same issue also featured a puzzle with British-made illustrations of Donald's nephews (along with Mickey's nephews):
(*) Donald's nephews would return to Donald's home in daily strips in 1938 for "just a few days", but stay on indefinetely.