Sarah Radfield Blechynden

Richmond Thackeray, was born at South Mimms and went to India in 1798 at age sixteen as a writer (civil servant) with the East India Company. Richmond fathered a daughter, Sarah Redfield, in 1804 with Charlotte Sophia Rudd, his possibly Eurasian mistress, and both mother and daughter were named in his will. Such liaisons were common among gentlemen of the East India Company, and it formed no bar to his later courting and marrying William's mother.

source https://en.wikipedia.org/wiki/William_Makepeace_Thackeray