Is there a grandfather paradox in Endgame?

During the film, Bruce explains that a grandfather paradox is impossible. We do get another explanation, that could explain what happens, from the Sorcerer Supreme.

Timelines can be split, we have our current timeline, where Thanos snaps his fingers and the Avengers go back in time to get the infinity stones.

The Avengers then interfere with that timeline, accidently, which would cause the timeline to split. Now we have two timelines, one where the Avengers never go back and Thanos snaps and one where they do go back and cause the conflict to move to the current year.

So instead of Nebula killing herself, she was killing an alternative timeline of herself. In the 2nd timeline, Nebula is dead but in the first she still lives on.

To quote the Doctor, it's all rather:

A big ball of wibbly wobbly timey wimey stuff

As Banner explains, they cannot change their past:

If you travel to the past, that past becomes your future, and your former present becomes the past. Which can't now be changed by your new future.

In other words, whatever change they do in the past will not ripple to the future. Thus, killing Past-Nebula does not translate to Present-Nebula being death.

In fact, they are two different realities. The Ancient One explains:

I'm sorry, I can't help you Bruce. If give up the time stone to help your reality I'm dooming my own.

So, how did the realities diverge? The Ancient One continues:

The infinity stones create what you experience as the flow of time, remove one of the stones and that flow splits.

The thing is, this already happened when Thanos destroyed the stones. That event allowed the realities to split.

Look at it in terms of parallel universes or realities instead of time.

Universe A is everything that we’ve watched in the movies up till now.

In universe A Captain America did not fight himself until 2023. He had no memory of fighting himself between 2012 and 2023. It was only after 2023 that he gained that memory. His going back to 2012 fighting himself created a parallel universe B. In universe B, Cap remembers fighting himself from 2012 onward. But that’s not our universe. We’re in universe A.

In our universe, universe A, Thanos comes from the past (of Universe B) to 2023. If Thor had not already killed him earlier, there would be 2 Thanos’ in universe A, one taking care of his garden (after the snap) and one fighting the Avengers. So in universe A Thanos has already snapped his fingers. His going from the past to 2023 and then dying could not alter what had already happened in universe A. Sure when he went to the future and was no longer there perhaps the snap never occurred but that’s universe B or C or D etc, not our universe which is universe A.

So by putting the infinity stones back at the same exact time they took them it is their goal to make all universes converge back to the near same realities as Bruce promised the Ancient One they would do. Maybe Cap in universe B remembers fighting himself in 2012 and Nebula in Universe C dies prematurely but the endgame is that all universes converge and stay on track so as to not drastically alter any one parallel universe. Remember Ancient One told Banner that fixing your reality (universe A) might throw mine (universe B) into chaos but by returning the stones exactly when taken that was avoided.

So in universe A, (our reality or universe) Steve Rogers married Peggy. We didn’t know it and Cap didn’t know it until after 2023 when he went back in time and stayed. We were always told she was married but was never told WHO she married. It was Steve Rogers. Otherwise he could never have been sitting on that bench in 2023 UNLESS we were watching universe B or C or D etc but we’re not. We’re watching universe A unfold, our reality.

Yes I agree, this breaks timeline rules. To answer your question; if Captain America goes back in time to live in the past and returns as an old man, then everything he did from 2012 onwards (as a young man) never happened. Thus breaking the timeline. Therefore this is a major flaw at the end of the film. Time travel storylines are always messy and contain flaws. This is a huge one.

How about this.

When the infinity stones are returned by Cap all the reality Bs disappear. Cap never fought himself. Loki never transported away with the case. Stark never talked to his father etc. In other words reality A was put back in place although each character would remember these events not from 2012 or 1970 but from 2023 when it happened to them. So Cap wouldn’t remember fighting himself from 2012 onward but only from 2023 onward. Howard Stark wouldn’t remember talking to “Howard Potts” in 1970 but Tony would remember it from 2023 until he died. And so on.

As for old Steve Rogers this is Reality A also. In Reality A Cap went back and married Peggy. There would be two of him: one frozen for almost 70 years and one married to Peggy laying low. When Cap visits old Peggy in Winter Soldier she has dementia and so what she says can’t all be taken as truth. But Cap couldn’t possibly know at that time that he was her husband. He wouldn’t realize he married her until it happened after 2023 events from his standpoint at the time.

See how it all fits. It’s all Reality A otherwise Steve couldn’t have been there on that bench.

But. Don’t forget that it is a linear timeline. In other words, Thanos snap came before his death in 2023. He still snapped first. Then traveled to 2023 which was after his snap. Therefore his snap still happened

Look at it in terms of parallel universes or realities instead of time.

Universe A is everything that we’ve watched in the movies up till now.

In universe A Captain America did not fight himself until 2023. He had no memory of fighting himself between 2012 and 2023. It was only after 2023 that he gained that memory. His going back to 2012 fighting himself created a parallel universe B. In universe B, Cap remembers fighting himself from 2012 onward. But that’s not our universe. We’re in universe A.

In our universe, universe A, Thanos comes from the past to 2023. If Thor had not already killed him earlier, there would be 2 Thanos’ in universe A, one taking care of his garden (after the snap) and one fighting the Avengers. So in universe A Thanos has already snapped his fingers. His going from the past to 2023 and then dying could not alter what had already happened in universe A. Sure when he went to the future and was no longer there perhaps the snap never occurred but that’s universe B or C or D etc, not our universe which is universe A.

So by putting the infinity stones back at the same exact time they took them it is their goal to make all universes converge back to the near same realities as Bruce promised the Ancient One they would do. Maybe Cap in universe B remembers fighting himself in 2012 and Nebula in Universe C dies prematurely but the endgame is that all universes converge and stay on track so as to not drastically alter any one parallel universe. Remember Ancient One told Banner that fixing your reality (universe A) might throw mine (universe B) into chaos but by returning the stones exactly when taken that was avoided.

So in universe A, (our reality or universe) Steve Rogers married Peggy. We didn’t know it and Cap didn’t know it until after 2023 when he went back in time and stayed. We were always told she was married but was never told WHO she married. It was Steve Rogers. Otherwise he could never have been sitting on that bench in 2023 UNLESS we were watching universe B or C or D etc but we’re not. We’re watching universe A unfold, our reality.