Cornwell (like most Ripper enthusiasts) tends to skate inconventient facts. To fit her theosry that the Ripper was impotent, she declares Sickett was too, on the basis he had some kind of operation at the age of 5. Ignoring that the guy was married three times.
Well, according to Cornwell, Sickert's early operation was to repair a fistula (literally a hole caused by a severe infection) in his privates and that the surgery was botched, rendering him "non-functional" physically even though both his normal biological impulses remained intact. Her argument therefore is Freudian in basis, but again, she has no real proof. However, Sickert apparently lived in White Chapel in 1888 and did have a morbid fascination with the Ripper in later life, even painting a picture entitled 'Jack the Ripper's bedroom.'