Lindsay Lohan -- New Half-Sister Is News to Me

BRAVO! Lindsay Lohan's such a brilliant actress ... she nearly convinced the world today that she didn't know about her newly-announced 17-year-old half-sister -- claiming, "I don't pay attention to any of that."

Of course, the news was everywhere yesterday after TMZ broke the story -- Lindsay's dad Michael Lohan took a paternity test for a 17-year-old girl named Ashley ... and the results came back positive.

But today on "Good Morning America" -- where Lindsay was trying to promote her new movie "Liz & Dick" -- she had the audacity to say, "I didn't even hear that."