Part 1 - Before Sarah is home
Tony and Steve had been married for 6 years now, had adopted their son Peter, for 4. Out of how proud of Peter they were; the pair wished for another, mainly this being Tony who wanted a daughter to spoil rotten. Steve also liked the idea of having a baby girl, so they made it official with the adoption of newly born girl. The young-ling came from a family who had been experimented on by HYDRA. They were trying to replicating the serum that was used on Steve back in 1942. It was suspected she would have abilities as the serum changed both the mother's and the child's DNA during the pregnancy, but there were no signs of her having any abilities after S.H.I.E.L.D. had run some tests on the tiny child. The girl had been taken in under their protection after S.H.I.E.L.D. had infiltrated the base where she was being kept. HYDRA had killed the parents to prevent S.H.I.E.L.D. from finding out what they were doing in this particular base. Steve and Tony adopted Peter due to a similar situation (his parents had been killed when he was at young age), and because of his spider like abilities it was best he lived with super heroes so he could learn how to control and use them. Despite anyone not knowing if the soon to be Sarah would have abilities, S.H.I.E.L.D. (particularly Phil Coulson) believed that she should live with the same super heroes to train her if they did come. This was perfect for Steve and Tony and took her on just the same as they did with Peter.
She would not be coming home yet for another week, but the couple had not told Peter about the fact he would be having a baby sister yet. Steve and Tony's fellow Avengers; Natasha, Clint, Bruce, Thor and a few other S.H.I.E.L.D Agents had known for months now about this and all were thrilled about the new arrival to the family. Still some had to tell Peter about the new family member. No one knew how to break the news to him though, let alone how he would react. Because of this Steve and Tony felt it best that the two of them told him with the rest of the family out of the house, if you could even call it a house?
But anyway; the family members that need to go each left with their own excuses to leave the two fathers and their son to talk things over. "Peter, Daddy and I need to talk to you" Steve said, sitting down next to Tony on the sofa with the boy cradled in his arms. He placed the child in the gap between the two of them. Peter's round brown eyes gazed into his Poppa's eyes. "We are having a new member of the family next week, aaand she will be your little sister" Steve said in a slight stutter. Steve felt it best to just come out and say it. They had been letting the news drag on now for too long and he need to get it off his chest. Tony was very good at sensing when things weren't right with Peter. He didn't really know how, he just did. He never really had a farther around to do things like that. It was always his mother Maria who did, so he chipped in with "Peter we still love all the same, but we just don't want you to get lonely by having no one a similar age to you to talk to". Tony just squeezed him gently into his body with Steve rubbing his back to sooth him.
After a few moments of worried looks exchanged between Tony and Steve, Peter pushed himself apart from his fathers. He spoke in a quiet voice and said "Don't worry Daddy I know you do." To both Steve and Tony's surprise Peter continued with "I can't wait to have a little sister it will be great, I can play with her, look after her, read to her and give her a hug when she is upset". The pair of the grown men took a deep sigh of relief and Steve hugged his two favorite boys in the whole world. This had gone a lot better than they imaged. Buried from within his father's Peter chirped "what's her name?" Steve smiled, rubbing his husband's back in comfort for himself. "Her name is Sarah, Sarah-Maria Peggy Stark-Rogers" Steve replied happily almost crying from the build-up of tears in his eyes because of how relived of his son's reaction and their little girl's perfect name. Both Steve and Tony named her after precious women in their lives; both their mothers and Peggy, one of the founding members of S.H.I.E.L.D and Steve's first lover.