Steve/Yelena
It was clear from the way he spoke of her that there was a lot of respect and care there. The offer to become a hopeful family as well took Yelena by surprise. He didn't know her the way he knew Natasha. Natasha's ledger had been wiped clean. Hers hadn't. But he was probably right. Natasha would have liked that.
"Well. I've never had a big brother before." She mused on this before something occurred to her and she let out a laugh. One that drew some attention for its unladylike volume, but Yelena didn't care so much. "HA! Alexei will be so pissed."
Steve spoke so impassionately about his character, she couldn't help but believe it. Or at least believe that Steve believed it. "People change, don't they? For the better, like my sister, or for the worse." She thought of the path of violence that Ronin - the dark persona Clint 'Hawkeye' Barton was known as - cut for the five years half of the universe was gone for.
A difference between messy and murderous. Yelena had little concept about that when it came to family. "Is there? A difference?"
Her perspective was skewed. The only family she had ever known had been fake and they had given her up to the Red Room at six-years-old. She had thought it was going to happen again when Melina hit her with her Widow's Bite and handed her over the Red Room a second time. She thought they were going to cut her skull open, with her still awake, to see what made her brain tick that she was no longer under their control. Even though they more or less made up after reuniting, it was still a sore spot under the surface.
"My employer gave me an assignment and told me he was responsible for Natasha's death."