I have posted MOST of what I know about Fanny within this thread.
She and her first husband Robert DAVENPORT were definitely Baptist missionaries to Siam. This is well documented and well established. I understand that they orchestrated the first Bible translation their and imported the first printing press.
There seems to be little doubt that she would have first hand knowledge of some events there, but I have no specific information about Siamese Twins.
Fannie was the daughter of Rev. David Roper (b 27 Sep 1792 - Charles City County, VA; d 28 Feb 1827 - Charles CIty County, VA) and granddaughter of David Roper (b 13 Nov 1744; d 16 Apr 1808 - Charles City County, VA). She was born in Virginia and has NO Siamese ancestors and as far as I know left no descendants or relatives there.
The published biography of Fanny's father Rev. David ROPER (from 1838) is posted at this thread:
"Biography of Rev. David Roper, of Richmond, VA" (14 Mar 2002 3:48PM GMT)http://boards.rootsweb.com/surnames.roper/461/mb.ashx