You were Natasha Romanoff's adopted daughter. She adopted you at 1 so she was always your real mother. Very few of the avengers knew who you were as Nat wanted to keep your life as normal as possible, though as you grew up and saw her on the news with all the Avengers, she explained everything. The only people who knew your existence was Nick Fury and Clint Barton. Your aunt, Yelena Belova also knew you, but she's not an Avenger.