I wasn't talking about hatred, I meant how are you going to convince her, and this is knowing that, by Crystalwatchers word, her guilt will come crashing down on her should the UD lose, not only from seeing that the UD's 'inevitable victory' was a lie, but also from the fact that a significant part of the UD winning was explicitly because of her own actions.
There many avenues to convince her.
First of all, the fact that her current position it's born from absolute desperation.
She's not with UD because she like it, if fact, if she could
really resurrect people to her heart content, UD would already been backstabbed by the gigantic army Demon Lord/Magical Girls/Angels/Whatever dead body Red found will going on a Phoenix Down spree.
We can use that.
She's with the UD because the UD is gonna win, but more than that is because she had(and has) no idea what to do.
If it was something as simple as "UD is gonna win" she would made a counter-plan, the problem is that is that she is the equivalent of an animal in a corner.
We can point that out, if we want to be brutal.
She joined the UD to save her family, and how is that going? Man what a good work, a brainwashed sister, the other two ready to kill, and she's completely suicidal, truly "Plan of the century" there.
Second, we bring hope with us, we can use that as a carrot.
Grey life, as long as Red was concerned was, pretty much "Puppet for life" and Red plan to resolve that was "Self-hate until the heat death of the universe".
But if she sides with us....She could actually have her family, the real one, back.
Third, we are an actual war asset no one considered.
Red considered the UD victory assured with Grey dead and White who knows where.( And the emotional impact of watching Hope burn like a mix of pepsi and mentos).
But now the UL has two goddesses plus the Old Guard plus the Drives plus us.
Things changed from Hope,
Fourth, a simple appeal to reason.
After the war she WILL be killed by the UD, she's too dangerous.
Even if the UD is winning, her only real hope of survival is to return to the fold before it's too late, because alone she won't have any chance to win.
Fifth, pointing out the sacrifices to make.
Is having a brainwashed Grey painful? That's nothing, she still need to kill White, and do the same thing to her, and the same with Gold.
If she remain with the UD, she will kill and resurrect her own family as brainwashed puppets.
Her "Victory" is simply a defeat where she can still fake some happiness instead of actually doing something.
The problem is not "There is no way to convince her" the problem "Is actually forcing her to listen to us without fighting".
Personally i'm more for the "Points out her error and flaws-->Shows her why her future is without hope even if she wins--->offer her a glimmer of hope-->Appeal to her sense of duty to her sisters if she fears judgment/appeal to her promise to the loves her sisters still feel for her" if we are in a stable situation for talking.
If not, i would start with the "hope" part to stop the incoming fight and then keep talking.