In the first Guardians of the Galaxy movie, Nebula (when she is firing on her sister Gamora) says:
"Out of all our siblings, I hated you least."
But then in Guardians of the Galaxy Vol. 2,
a large theme of the movie is Nebula's intense hatred for Gamora.
Is the quote from the first movie just a throwaway line then, or was there a fundamental shift in their relationship? In-universe or out-of-universe answers welcome.
First of all, I've only seen the movies. I have no clue, if the comic gives more information about this. Second of all, I don't know the other siblings.
To your question:
"Out of all our siblings, I hated you least."
still implies, that Nebula hated Gamora.
We get the reason for that in Vol. 2. Gamora and Nebula had several fights with each other where the winner remains untouched while the loser gets a part of her body replaced by some mechanical device in order to make her a better fighter. As the time went on, Nebula lost every single fight and thus turned into a cyborg. This, of course, is linked with a lot of pain and the normal living being tends to dislike and later hate whatever is the source of it. This also seems to be the only reason for Nebula, as she stopped attacking Gamora, after she considered herself as the better fighter, reasoned by the fact, that she just defeated Gamora in a fight.
Despite all the this, Gamora seems to be a nice person and always treated Nebula rather nice. As long as Nebula didn't attack her, that is. So while they had their differences, the other siblings of them, which we don't know as of now from the movies, may have treated Nebula even worse. Remember, words can hurt you far more than any form of physical discomfort. And I can easily imagine for sons and daughters of Thanos to be rather rude in their communication and interaction with others.
Edit:
As a addition, why Nebula's hate seem to increase in Vol. 2:
Remember that she was part of Ronan's Crew in the first Volume, meaning she actually had something to do. But with the death of him at the end of the first Volume, she lost that part. Unsure what to do, she became a simple mercenary, taking a job to steal the batteries the Guardians are protecting at the beginning of Vol. 2. (If I remeber correctly, that is. But I can't remember any other reason why she would do that despite being mercenary =/)
After she got on the GotGs ship, she figured, that it was time for her to do what she wanted to do her whole life and follow her ultimate goal. Imagine a simple staircase. At the top lies her utmost wish:
killing Thanos.
On her way she has to clear things with her siblings. The first stair for her to step on to is Gamora. Nebula's grudge on her can simply be lifted by a single fight, reasons for that already mentioned above. While being weaker or stronger than somebody else is a rather meaningless thing for you and me, it means the life for Nebula, as literally her whole life were linked to these fights and winning just one would break the cycle and remove a huge weight on her shoulders as it would mean, that she got stronger and thus closer to her goal.
The reason why you think that the hate intensifies are the meanings by which Nebula tackled the confrontation. Then again, recall that Nebula lost every former fight. This continues even in Vol. 1. So she figured, that she would've to rely on some harsher ways of attacking her, resulting in what we have seen in the movie.
In Guardians of the Galaxy Vol. 2, Nebula says that she wanted a sister, while all Gamora wanted was to win. This was the root of her hatred.
GAMORA: Nebula, I really don't need this. My day has been bad ENOUGH
NEBULA: I don't need you always trying to beat me!
GAMORA: I'm not the one who just flew across the universe because I wanted to win!
NEBULA: Don't tell me what I want!
GAMORA: I don't have to because you make it obvious NEBULA: YOU WERE THE ONE WHO WANTED TO WIN, I JUST WANTED A SISTER!
In Avengers: Endgame, Future Nebula and past Gamora have this exchange.
GAMORA (2014): Tell me something. In the future, what happens to you and me?
NEBULA: I tried to kill you. Several times.
NEBULA: But eventually, we become friends. We become sisters.
In the end Nebula gets what she always wanted, to have a sisterly relationship with Gamora.
