According to my notes Arthur did have a sister named Sarah.
They were children of John Giles (son of William whose will is dated 1692 and Betheniah Knowles Giles).
John married Mary Moseley and they had:
1.Arthur, b.1725 2.William, b.1727 (m. Ann Branch and they were the parents of the Virginia governor, William Branch Giles) 3. Sarah, b. 1729 4. Nicholas b. 1733 (one of at least three Nicholas Gileses; John yougest brother was a Nicholas and he had a son, Nicholas, Jr., perhaps a forth one, too) 5. Mary, b. 1734