Gamora had watched everything unfold on earth and knew it was because she hadn't been able to stop him. She couldn't understand how he'd got the soul stone, but now she knew. There were some things she wasn't willing to sacrifice, Nebula had suffered so much at his hands, before of her. Obviously it had been too much, her sister's pain after a lifetime of abuse.
As little girls she remembered holding hands, playing games and sharing secrets. But that was so long ago, to see her reach out for her, it reminded her of that innocence. Part of her was still unsure, this could all be some kind of trick and that was why she hesitated. But after a beat Gamora took her hand and something in her settled, the fear and the itch to run from it all.
"You are my sister," Gamora said as she squeezed her hand. "I should have a long time ago."