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 <PERSON> hated <PERSON>.
We get the reason for that in Vol. 2. <PERSON> and <PERSON> 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, <PERSON> 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 <PERSON>, as she stopped attacking <PERSON>, after she considered herself as the better fighter, reasoned by the fact, that she just defeated <PERSON> in a fight.
Despite all the this, <PERSON> seems to be a nice person and always treated <PERSON> rather nice. As long as <PERSON> 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 <PERSON> 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 <PERSON> to be rather rude in their communication and interaction with others.
Edit:
As a addition, why <PERSON>'s hate seem to increase in Vol. 2:
Remember that she was part of <PERSON>'s Crew in the first Volume, meaning she actually had something to do. 865
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 <PERSON>.
On her way she has to clear things with her siblings. The first stair for her to step on to is <PERSON>. <PERSON>'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 <PERSON>, 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 <PERSON> tackled the confrontation. Then again, recall that <PERSON> 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. 865