Dear Bruce
As to why the 5 Austin daughters and 1 brother went to Madras/ Ceylon (now Chennai/Sri Lanka), I think most can be explained in terms of who went first ..
Ceylon: probably Mary Ann (1806-75) went first, marrying William Morris - do you have details of their marriage? I have a note to look in Blackwoods Magazine of 1829.
Martha (1817-1870s) and William (1811-48) presumably were influenced by their being there: Martha possibly husband-seeking - she met George Lee on the boat out, marrying him later that year (1841) in Kandy; and William to seek his fortune as a coffee-planter.
The question we still haven't answered is whether Mary Ann meeting William Morris was because of a relationship to Dr William Austin (1790-1871), who married William M's sister.
Still have no info on where the father James Austin came from.
Madras: Susanna Sophia (1804-53) went out to Madras in late 1833 with her surgeon husband William Liddell to become Assistant Coroner there, on the Wellington, a (very interesting) ship captained by his brother James Liddell. They took her sister Sophia (1808-67) who later (1842) married Rev John Garrett - I have a memoir of their family by their daughter Mary Sophia Rice.
Finally, Eliza (1819-70s) went out presumably to stay with the Liddells until she married the banker John Innes Geddes there in 1849.
The theme common to probably Sophia, Martha and Eliza, of going to India/Ceylon to seek a husband, is a well-known phenomenon. I guess it appealed to a certain section of society, reasonably well off daughters of large families ??
So the main question remains - were James and William Austin related?
By the way, I do have more info on James's wife, Sophia Hill, and her family, which helps explain how James ended up catering for the Guards:
From the Gentleman's Magazine, for 1820:
6 November: "In Park-street, Windsor, Adam Hill, many years Mess-master to the Officers of the Brigade of Guards."
Adam's will confirms the relationship.
Best wishes
Denis