DISCLAIMER: Nothing in this is based on the movies. Only little parts, but not everything.
You're the younger sister of Steve Rogers. You got the same super-soldier serum and you were on the side of your brother his and your entire life. Since 1920 you've had the biggest crush on his best friend, Bucky. After you heard of his death you were devastated. Soon crashing into the ice for 70 years, later to be discovered. S.H.I.E.L.D. soon found you and you've been one of the Avengers ever since. You have mixed powers with Wanda and Doctor Strange, you're a tiny bit stronger than Thor, making you the strongest Avenger.
Around the late 1930s, you and James Barnes were engaged, almost about to get married. Until he was presumed dead in the 1940s.
At present time, we haven't found Bucky (TWS) yet and you're dating Loki Laufeyson. Natasha, Wanda, Peter, and Thor are your best friends.