Can we all just imagine Gamora and Nebula being friends and actually having a sisterly bond.
HEADCANON:
When Gamora's parents were killed by Thanos and Thanos took Gamora as his daughter Gamora despises the idea of living with him and her new siblings.
It takes Gamora a few weeks to get used to living with Thanos, Nebula, and Korath instead of despising it. After she gets used to it she starts to befriend her siblings instead of just saying "Hi." or "How are you?" every time she runs into one of them. And she and Nebula happen to create a strong bond which lasts a couple of years. Then Nebula starts doing shocking, rude, and annoying things which really starts to annoy Gamora. Yet Gamora hides her feelings for Nebula's new behavior for a few months, but she can't hide it forever, which starts to tear apart the sisterly friendship. And Gamora stops spending so much time with Nebula which leaves Nebula questioning why their bond isn't near as strong as it used to be. Awhile later Nebula starts to gain hatred for Gamora from all the sadness, anger, and pain her sister caused her. Gamora gains the hatred for Nebula as Nebula did for Gamora. Which leaves the sisters to this day:
Despising each other so much to where their enter relationship turned into a giant ball of hate.