This idea somehow popped up in my head and wouldn't leave. Tony Stark has a baby sister whom he raised after his parents died. She is his biggest secret. Now she's graduating.
Just to get the time line straight (or at least I think that's how the MCU timeline could go): Iron Man 1&2=2008, Avengers+IM3=2009, Captain America: Civil War+ SpiderMan Homecoming= 2017 (not that this last one actually matters, but it fits).

My little girl

"I can't believe she's graduating. She was a baby just a moment ago."
"You did a good job raising her, son."
"I just did what my mother did and all the things my father didn't do. I couldn't have done it without your help, Aunt Peggy. Thank you." Said Tony Stark as they turned their attention to the stage. Peggy took his hand. Somehow they both managed to get by unnoticed by the crowd.
Most of the graduates were the same age and then there was Charlie. At 18 years old Charlotte Peggy Carter-Stark, the spitting image of her namesake, was graduating MIT in Physics and Mechanical engineering. Her big brother and aunty couldn't be more proud of her. Tony smiled to himself as he remembered the sleepless nights walking around his workshop with a crying baby in his arms or how she held onto Dummy when she was learning to walk.

Everything started that one day on July 7th 1991, the day she was born. Maria really wanted a daughter, a little girl to spoil and dress, teach the piano and sing with her. His parents had a surrogate, because his mother was too old to give birth. At least that's what he's been told, but he found later it wasn't the whole truth. His mom couldn't have kids. She couldn't but Aunt Peggy could. Dad never hesitated to ask his friend for a favor to make Mom happy. Peggy agreed. That's how Tony was born and then 21 years later, with the help of IVF and the genetic material from my dad and Peggy, Charlotte came to be. His mom was so grateful to Peggy she gave Charlie her name.

His mom and Dad died the same year in December. They were going to the Bahamas and had a car accident. Charlie was with Peggy for Christmas. She came to find him as soon as she could, but he already managed to do some things to avoid grieving that he's not proud of. When Tony saw her with Charlie, he knew he had two options. Give Charlie up or raise her. Peggy said she could take her, but she was still working at SHIELD at the time and it would have been dangerous. When Tony looked at his baby sister, he knew there was no way he could give her up and not regret it for the rest of my life. She and Aunt Peggy were the only family he had left. The world never knew about Charlie and it was better that way. Tony wanted her to have a normal childhood that didn't involve being chased by reporters. With Peggy's help they created a new identity for her as Charlotte Carter to be used during her school time. It worked out and Charlie was safe. That was 18 years ago. To this day no one beside him, Peggy, Rhodey, his other bestie Emily and Pepper knew about Charlie. He trusted Pepper with his life, so why not with his sister.

There he was sitting next to his biological mother on his sisters' graduation. Captain had been pissed when he said he would ditch yet another boring team building activity and do something else instead. One day, when he would trust them enough, he would tell them the truth about his baby girl. But not yet. Pepper had been understanding and cleared his schedule so he could enjoy this important day. Too bad she could not make it. Charlie had just walked up on stage and as the top of class she was asked to give a speech.
"My brother once told me that there was no point in doing things you don't love and he was right. Every one of us is here today, because we did just that and probably because the Avengers stopped those aliens in New York. I don't think we'd be standing here if they didn't.
We can't know what the future holds or where we end up in a couple of years. Many of us know, or at least claim to know, the next step they'll be taking. Others have no idea. In the past years we learned not only how not to blow up a lab, although accidents still happen, but we learned a lot about ourselves as well. The biggest thing I learned and I'm probably speaking for a lot of us here, is that we only get one trip around this rock, so we might as well enjoy it.
You have to find a balance between work and fun. A well-rounded person is someone who knows a little bit about every topic his or her friends discuss, and an interesting person can take that info and have a conversation about it, even if his or her knowledge is lacking.
To everyone who touched our lives, we say thank you. To parents, who stood by us no matter what. To teachers who tried to instill in us a passion for learning. To administrators who wanted to make our college experiences as amazing as possible. To advisors who helped us decide what path to take. To friends who were there for pizza parties, study sessions, and everything in between. And to all those others we depend on. Without you and without each other, there would be no purpose to being here, no way to advance. Life, like society, is exactly what we make of it."
Charlotte looked at the audience, took a deep breath and continued: "We all have people here whom we owe a lot at without whom we wouldn't be standing here. Personally, I'd like to thank the person who raised me and that was my older brother. Without you I wouldn't be here and you taught me the most important lesson of all. Just because something is hard, it doesn't mean it's impossible. And now, Class of 2009 toss those hats in the air, because we did it!"

Hats flew in the air and the crowd was clapping. When his little girl ran up to hug him, tears of joy escaped his eyes. "Thank you for everything you did for me, Tony." she said. Tony Stark never felt prouder...