The Mean Girls star's dad, Michael Lohan, took a paternity test during an appearance on U.S. daytime programme The Trisha Goddard Show on Thursday afternoon (15Nov12) - and medics confirmed he was the father of his ex-girlfriend Kristi Horn's 17-year-old daughter Ashley.
Michael failed to break the news to Lindsay, who is on a promotional tour for her new movie Liz & Dick, and the actress missed all the news reports about the big TV reveal. She was left stunned when a reporter on the breakfast show asked for her opinion.
Journalist Amy Robach said, "Your family troubles are constantly put out there for the public. I know just today there was news that you allegedly have a new half-sister..."
But the shocked 26 year old interrupted her, saying, "I didn't even hear that, so thanks for the news."
Robach continued to pry, however Lindsay smiled and handled the situation with grace, adding, "I don't really want to get into that because I want to stay on the positive side of things."
The Good Morning America interview marked Lindsay's first candid TV chat since a series of headline-grabbing events over the summer (12) - she was due to sit down with Barbara Walters for an upcoming appearance on news show 20/20, but axed the gig over concerns from her public relations team the U.S. broadcaster would delve too deep into the star's personal issues.