When writing her book The Life and Lies of Albus Dumbledore to present a skewed yet truthful history on Dumbledore's life, Rita spoke to people who had known Dumbledore when he was young. She tried to interview Elphias Doge, but was rebuffed. She spoke to Grindelwald's great-aunt, Bathilda Bagshot (using some highly questionable interviewing methods). She spoke to an old neighbor of the Dumbledores, Enid Smeek.
Yet she didn't speak to the one person who knew Dumbledore better than anyone, who knew the truth about Ariana, and Kendra, and Dumbledore's friendship with Grindelwald. She didn't speak to Aberforth. Given Aberforth's rift with Albus, the way their fight at the funeral was described, you'd think she'd have really wanted to interview him for her book. But she didn't. Why?
Maybe because Aberforth knew the actual truth, not rumors and speculation, and while it may not have put Albus in the best light, the actual truth was the last thing Rita was interested in.
Either that, or maybe Rita did find out who he was and track him down, only the moment she started asking questions about Ariana, Aberforth kicked her out.