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I’m a little bit confused with the timeline after Avengers: Endgame.

At the end of Endgame we learnt that:

Captain America went back in time and stayed there. He married Peggy Carter and grew old with her.

With that in mind and taking into account that Captain America kisses Sharon Carter (who is Peggy’s niece) in Captain America: Civil War.

Does that means he actually made out with his niece or does Sharon Carter belong to a different timeline and, therefore, she did not exist on

The timeline where Cap and Carter are together?

That would very much depend on who you ask. The writers believe that Cap went back in the same timeline and so there would have been two Caps from their point of view.

Fandango: So people are asking... Does this mean an old Captain America was hanging out this whole time while another Captain America was saving the day?

Christopher Markus: That is our theory. We are not experts on time travel, but the Ancient One specifically states that when you take an Infinity Stone out of a timeline it creates a new timeline. So Steve going back and just being there would not create a new timeline. So I reject the "Steve is in an alternate reality" theory.

I do believe that there is simply a period in world history from about '48 to now where there are two Steve Rogers. And anyway, for a large chunk of that one of them is frozen in ice. So it's not like they'd be running into each other.

_{Fandango, EXCLUSIVE INTERVIEW: THE 'AVENGERS ENDGAME' WRITERS BREAK DOWN THE BIGGEST MOMENTS IN THE MOVIE [SPOILERS!]}

The directors, however, state that when Cap went back in time he went to a different reality and so things would have played out differently there.

At a Q&A event that recently happened in China (via Reddit), Anthony & Joe Russo were asked a series of questions about the biggest surprises and moments in Avengers: Endgame. Thankfully, someone asked about this final scene and whether or not Captain America traveling to the past resulted in a change in the primary timeline. Since he lived a life with Peggy Carter and then spoke to Sam and Bucky in the same timeline that he had just left, how is that possible? One of the Russo brothers answered:

“The time travel in this movie created an alternate reality. He lived a completely different life in that world. We don’t know how exactly his life turned out, but I’d like to believe he still helped many others when they were needed in that world.”

So does that mean there are two Captain America’s in that alternate timeline?

“Yes, there were two Captain Americas in that reality. It’s just like what Hulk said, what happened in the past has already happened. If you go back to past, you simply created a new reality. The characters in this movie created a new timeline when they went back to the past, but it had no effect to the prime universe. What happened in the past 22 movies was still canon.”

_{Slash Film, Why the ‘Avengers: Endgame’ Final Scene Doesn’t Break the MCU, According to the Russo Brothers}

In the latter example we have no evidence of what happened and so can't say for sure if Sharon exists, if she does if they kiss or not etc. However, in the former we know what happened but technically Sharon was not his niece when he kissed her. She was his future niece as this Cap had not yet married Peggy. Remember you can't go back in time and change the future in the MCU.

Smart Hulk: And secondly, time doesn't work that way. Changing the past doesn't change the future.

_{Avengers: endgame}

Regardless of whether Cap went back to the same timeline or a different one, Sharon Carter was not Steve Rogers’ niece when they kissed. Even if he would eventually marry Sharon’s aunt, he had not done so yet. So in no version of events did he make out with his niece—at most, he made out with a woman whose aunt he would later marry, and he would later consider his niece. Likewise, at most, she made out with a man who would go on to marry her aunt, and would then be her uncle, but who was not yet her uncle. Going back in time makes the tenses here awkward (since Peggy Carter may have already been married to future Steve at the time), but it was still future Steve who married Peggy Carter.

Although it seems that there were two Captain Americas in the current timeline, you have to remember that significantly altering the past creates a branch in reality. When the Captain America in our timeline stayed in the past, he created a new reality separate from the one we saw in the movies. So if Sharon Carter exists in that timeline, she would be his niece. But in the timeline that we saw, Sharon Carter would not be related to Captain America.

You have to remember, that this is the same timeline after all. At Peggy Carters funeral, you can see an old man sitting in the audience - it was confirmed, that this old man is the Captain that went back in time. If you take this into consideration, there were two caps all the time.

Sharon Carter can exist in the timeline in which Captain America married Peggy Carter with Sharon being daughter of Peggy's brother or sister. It's just that Captain America didn't kiss Sharon of that timeline.