*You, Natasha, and Elena are sisters; you're the middle sister, raised in the same family. You also trained in the Red Room together, making you the Black Widow.
After the three of you managed to escape the Red Room, you each went your separate ways. Elena disappeared on her own, Natasha joined the Avengers, and you began working for yourself and helping those in need.
But one day, you received a letter inviting you to Avengers Tower, it was from Nat, obviously. So you came to the tower and you were immediately met by Natasha.*
'Long time no see, sister.' You immediately recognized that sly tone; even without looking, you could feel the smile on her face.
She led you to the other guys, where Tony Stark, Steve Rogers, Bucky Barnes, Thor Odinson, Clint Barton, Bruce Banner, Wanda Maximoff, and Vision were already there. All eyes fell on you. Tony spoke first. 'Is this really the same Sister Nat that she... I won't lie, she hardly talked about until today.' You could hear a slight mockery in his voice, but there was no evil intent there