Isabella joined the Auxiliary Territorial Service (ATS), the same branch of the Services as her sister Katherine.
She probably took the same route as Katherine to the Meeanee Barrack in Colchester,
then onwards to Saldean, where she would have been officially enrolled into the ATS.
She advanced to the rank of Lt Corporal serving in the UK and abroad.

The Newspaper article below, refers a marriage to Gunner Robert D. Caskey (Royal Artillery).
Ancestry list them married in 1940 at Shardlow, Derbyshire.

Interestingly, the Royal Horse Artillery were stationed here in Bures during 1940/41

I was unable to find any Military Service records online for Isabella and Robert.

Ancestry records indicate she later married Clifford Duckworth, but I can find no date when this took place.
So what happened to Robert ?
Isabella & Clifford had two children.

Isabella survived the war and died in Blackburn 2nd February 1995.
Probate confirmed her name was Isabella Aitken Duckworth.

Photograph enhanced from newspaper above.

According to Cameron Carr, his sister Elizabeth enrolled in the NAAFI.
The NAAFI in Colchester was located at the Artillery (Le Cateau) Barracks,
so it would seem plausible she chose to work in the Barracks where Katherine and Isabella had briefly attended.

Unfortunately like Isabella, I can find no Military Service records for Elizabeth.
However, she is documented below as Manageress of the NAAFI

On the 23rd April 1944, Elizabeth married Paul Garrow at St Marys Church in Bures, she survived the war and subsequently died in Birmingham 1993.