If you asked Steve ten years ago who his soulmate was, he'd say James "Bucky" Barnes in a heartbeat. They were inseparable, having been best friends since childhood and in love since high school. They did everything together, truly in sync, with the most love anyone had ever seen. They married, had a beautiful son, and were supposed to grow old together. Supposed to. That was before he lost him.
If you asked Steve two weeks ago who his soulmate was, he'd say Sam Wilson without thinking. They had a bond unlike any other, working together to build a life from what they had both lost. Sam is a gift in Steve's life, someone who pushes him to be a better person, father, and husband, while also loving Steve for who he is, just as Steve does to him. Out of the military, they're making something of themselves as students and fathers to Grant, Steve and Bucky's son (and now Sam's), and Faith, their precious little girl.
If you ask him in two days who his soulmate is, Steve wouldn't know what to say. How do you choose between the man you used to love but lost and the man who gave you everything you didn't know you needed?
You are one badass assassin. The deadliest one alive. You're also Steve Rogers' long lost younger sister. When Steve and the Avengers find you single handily taking down one of their targets, they invite you to live with them. You only agree because Steve is there and you want to be with your brother. You don't know Bucky is alive until you get to the tower. Bucky's been in love with you since the 40's. He would constantly beg Steve to let him marry you. Steve would refuse. You admit that you wouldn't mind being with Bucky since you've always felt something for him back. The only thing standing in yours and Bucky's way is, well, Steve. So what else can you do besides have a secret relationship and hope Steve doesn't find out.
This takes place after endgame and infinity war, but Thanos never happened and The Avengers are living together.
I do not own any of these characters (besides y/n) and they all belong to marvel.