When Celestia discovered her virtue and abandoned the natural path of a matured alicorn, Hyperion was disgusted with her choice. He didn't know what to do. On one hoof, she was still an alicorn. But on the other, she was more like one of those insects. He could not reconcile the two. He could have destroyed her right then, but he had invested much in her, and he could not bring himself to end her life.
Instead, he focused on trying to convince Luna that the mortal ponies were not worth throwing her life and future away for like her sister did. She listened to him and weighed his arguments. Luna had always been the more mysterious of the two. She would silently judge you, watching and waiting for the right moment to present her conclusions.
Luna weighed what Hyperion told her, but also what he showed her through his actions. She also watched her older sister and the changes she underwent. Luna was only sixteen years old, and she had a lot of decisions to make.
Seven years later, Hyperion had shown his true colors as far as Luna was concerned. Time had proven that Celestia had made the correct decision. She had become a ruler that the mortal ponies appeared to really need. No, not merely need—they actually wanted her. In some way, Celestia's strange ramblings about the mortal ponies being real people started to make sense.
Luna began to question everything she was raised to believe. She knew, even as far back as her memories would allow, that Hyperion was unjust in his treatment of the mortals. But she never saw anything wrong with her mother's treatment of them until some rather long talks with Celestia in her late teens.
At the age of twenty-three, Luna made her decision. She had seen her father's unjust treatment of the mortal ponies, often killing them in the most cruel or humiliating ways possible. Celestia advocated on their behalf, urging Luna to recognize that they were actual people and worth defending. After much consideration, Luna discovered her own virtue of Righteousness.
The transformation was startling. She knew from watching Celestia over the past seven years that she would cease to gain further alicorn gifts and powers. But Celestia did not seem to mind. For Luna, it was a jarring experience. She knew the magnitude of what she would be giving up, but it was still the right thing to do, to undergo the change. Once it was finished, Luna became the champion of justice for the mortal ponies. She ruled alongside her sister as fellow-advocate and defender.
Meanwhile, Periapsis was proud of her daughters in her own way. While she did not see the ponies as actual people, she still cared for them and spent much of her time keeping them because it pleased her. She did not understand the depth of connection that her daughters had with the mortals, but she did not condemn it either. She found it a bit amusing, and also a bit overreactive, to toss their entire lives into the mortal society, giving up so much in the process.
But she was not angry like Hyperion. He was truly outraged. His prize daughter, Luna, had essentially given him the worst insult imaginable. Hyperion grew increasingly bitter and hostile toward Periapsis and eventually their children as well. He eventually reached a breaking point with his family's interest in mortal ponies and began actively destroying their civilization.