JK Rowling’s estranged father secretly ran a large-scale marijuana-growing operation from her childhood home in southwest England during the 1990s, while the future “Harry Potter” author was a struggling single mother in Edinburgh, Scotland, a new book claims. The allegations about Peter Rowling, 81, appear in “A Pen to Change the World,” a new biography by Solomon Schmidt to be released next month. “When Pete was visiting me at one point in 1997, he told me that over the previous few years, he had been one of the largest growers of marijuana in South West England and Wales,” Peter’s nephew, Ben Rowling claims in the book. JK Rowling (centre) with her father Peter Rowling and step-mother Janet. PA Images via Getty Images A never-seen-before picture of Peter Rowling in 1977. Courtesy of Solomon Shmidt According to an excerpt obtained by The Post, Ben claims his uncle started the illegal operation in the years after the death of JK’s mother, Anne, in 1990 after a ten-year battle with multiple sclerosis (MS).…