Foster Sister - what does it mean?

[chempat]

Sorry, that was the second one down that I ignored

Husband is William Allan Johnston

[hanes teulu]

I wonder whether “foster sister” in this context merely indicates that the young ladies grew up together, the Brownlee girl being associated with the royal (ducal?) household due to her father’s position; perhaps they shared a governess or similar?

I had formed the same impression having read of the relationship between Napoleon III and Madame Cornu (Hortense Lacroix) " ... the foster sister, the early playmate and the almost lifelong confidant of the last emporor of the French ....".

Here she is described as "filleule" (god daughter?) -
http://data.bnf.fr/16183183/hortense_cornu/

Iolaus,
The article is available via British Newspaper Archive (BNA) or FindMyPast (FindMyPast) - both subscription/credit sites

[stanmapstone]

From the OED
ˈfoster-sister A female child nursed at the same breast  as, or reared together with, another of different parentage.

Stan

[chempat]

The Princess was born in November 1876 and left Malta after 2years to go to Kent.  She later returned to Malta in 1886.
Elizabeth Victoria was born in Malta (1879), her younger siblings (Marie and Maude) were born in Edinburgh, so when would they have been 'foster-sisters'?

http://royalcentral.co.uk/blogs/insight/the-stories-of-queen-victorias-granddaughters-princess-victoria-melita-of-edinburgh-51689

[avm228]

Elizabeth Victoria was born in Malta (1879), her younger siblings (Marie and Maude) were born in Edinburgh, so when would they have been 'foster-sisters'?

Perhaps in the 1890s, in Edinburgh?