“Won’t you try some cookies, my dearest cousin? I made them especially for you.”  
“Go eat a booger”
 
As children Fire Opal (who changed her name post-cutie mark to Fire Aurora) and Twilight Moonstone did not like each other. Moonstone thought Opal was a selfish brat, and Opal thought Moony was a stuck up holier-than-thou know-it-all. Unfortunately, they are about the same age, and were thus forced to play whenever their families would visit each other. But they couldn’t be openly hostile towards each other, that would just net them another lecture on family, and harmony, and blablabla. So they settled for pranking each other and making thinly veiled insults, while acting like it was just joking around whenver an adult was looking.
 
As adults they still don’t very much like each other, but it’s more of a not caring about the other, rather than open dislike. In a way, they are actually closer to one another than most other people they know, as they know every weak point, and how to get under the other’s skin. They are also really good at working with each other, since they’ve long since learned how to wordlessly coordinate in order to avoid their parents finding out about their hidden war.