By Lauren Haughey A devastated woman has shared the gut-wrenching story of her boyfriend’s affair, with the internet stunned that he had cheated with her own brother’s girlfriend. The anonymous 24-year-old said it was finally time to ‘get it off her chest’, warning readers to get ready for a ‘novella’ as she delved into the cautionary tale. Taking to Reddit, she explained that she’d started dating a man called Allen at 23. Although she’d known him since nursery, the pair had never actually gotten along very well until one fateful Spring. “We quickly entered a relationship,” she wrote. “Looking…