May well be 2 fics but what I remember is Tony is in hospital, post Siberia and Sharon visits. She mentioned kissing Steve, not knowing that Tony/Steve had been a thing at the time. They had not told anybody.
Other part (or fic) is Bucky not happy to find out that Steve was in a relationship with Tony and kissed Sharon.
By Our Own Hands by poetically_ordinary
Unfortunately orphaned and unfinished, both parts were same fic but Tony was in hospital again after being attacked by Wanda.