This is what I think went on with the other three Diamonds relationships with each other and Pink. Disclaimer I own nothing.
As a Diamond it is expected of you to be calm, commanding, respected. You can't show weakness, you can't cry in front of your Gems unless it's in front of Gems that can't tell everyone like a Pearl or Zircon or in front of a fellow Diamond. If you show even the faintest hint of weakness you run the risk of Gems becoming weak themselves. Yellow understood this best, she learned so quickly that it was almost terrifying. She was a terror in battle; she took to commanding other Gems in battle instantly. I gave her full command of the military within a year and a little after that she had her first colony. Of course I didn't show how proud I was until she reported that the colony was complete. Showing too much pride runs the risk of one wanting it all the time. But perhaps I showed too little pride… it used to be that Yellow would contact me every time she wanted to report a small victory and though I wouldn't show it I was pleased to hear from her but over time she'd only call when she'd pick a new planet and when that planet was colonized then she'd only call to say she completed a colony. After what had happened with the Earth Yellow then ceased all contact with me and would instead send me reports about new colonies like she was just another one of my Gems. I'd call her and we'd talk for about a minute before she'd hang up. Last I heard Rose Quartz was brought to Homeworld for a trial and Blue was apparently supposed to tell me about the trial. There was nothing else, no explanation on how it happened or anything just that there was a trial. I miss her reports.
Blue Diamond was more complicated; she had a powerful personality though she was more willing to show emotion. I didn't make her for battle I made her a diplomat, it took a while longer than Yellow to get Blue ready for her first colony. I tried to instill an air of emotionlessness in Blue but it never completely stuck. I was strict with her; I always pushed her to do all things with absolute perfection. But maybe that was the problem, she was once eager to talk to me but when Blue began getting colonies she limited our talks to whenever she'd pick a colony and when she'd finish it then she'd only call to tell me in person that she finished a colony then she'd send in reports and now she has her Gems send in reports. I try to call Blue and even leave her messages but she's always too busy to respond. Blue always used to bring her problems to me, now I'm lucky to hear of those problems thousands of years after they've been solved. When I heard about the trial I asked her why I wasn't told about it and she said that only those who cared about Pink Diamond benefited from it. She then went on to say that the only reason I helped with the corruption blast was because some Gems rebelled and not because Pink was shattered. Blue didn't let me respond, she left the conversation after saying her piece. I wish I could say sorry to her.
I guess deep down I missed having a Diamond constantly ask me questions so I made Pink on a whim, first mistake. She was so small compared to us but that's ok, a Diamond just has to lead. However while the other two Diamonds could keep their emotions in check Pink was a big ball of energy. I didn't show it but I enjoyed the time we spent together. I thought I was doing rather well; I didn't push her away at the very least however I could tell that I just wasn't showing her the right kind of affection. Then Blue and Yellow brought her to me, Pink was requesting a colony and it was a rather interesting request. Pink explained that since she was allowed to live then other defective Gems should live perhaps on a planet that none of us wanted. Pink felt that off colors and defective gems might be able to serve some purpose so she wanted a colony full of them so that she could figure that purpose out. Yellow seemed a little interested in this idea and Blue was looking at me expectedly. I'll admit it is an interesting idea but as I told her we can't afford to waste time or resources on off colors or defects. Gem instinct forces us to need to constantly reproduce. I just sat there as Pink's face fell, I could hear her whisper, "Then am I a waste of time?" Blue's hood covered her eyes as Yellow looked down and apologized for wasting my time. After that day Blue had her visit a lot less and that was just to sit in on the meetings. I wanted to tell her how sorry I and that her idea would be implemented someday but something inside me prevented me from speaking. The last time I saw her I let her have the Earth. Then I heard she died in a war I somehow didn't know was taking place. She didn't even want me to help her! After that I closed myself off, this whole thing happened because I was listening to my feelings. Emotions are a useless distraction. But then… why do I feel so empty?
Hope you liked it.