In Yukino Yukinoshita's view, Sogo Doma's circumstances were truly very similar to her own. They were both born into prestigious families—though the current Yukinoshita Family was far inferior to the Doma Family, the gap between their families had not been so great in the past. They were both the second child in their family, and both had an almost perfect older sister—or brother—above them...
It was precisely because of that that Yukino Yukinoshita believed she understood Sogo Doma. She even felt that she could understand why he had developed such an awful personality.
After all, with an older brother—or sister—who was practically perfect above them, no matter what accomplishments they achieved, people's first reaction would only ever be—"As expected of Yukinoshita Haruno's little sister," or "Taihei Doma's little brother."
It was as though every achievement they made was simply because they were Haruno's little sister or Taihei's little brother. No one cared whether they had worked hard, because they were Haruno's little sister or Taihei's little brother. Everything was only natural.
Faced with this kind of "special treatment," anyone... Well, Yukino Yukinoshita certainly couldn't accept it. However, she wanted to surpass Haruno through her own efforts, so that when people saw her alongside Haruno, they would no longer see her as Haruno's little sister, but instead say to Haruno, "As expected of Yukino's older sister!"
And in Yukino Yukinoshita's eyes, the reason Sogo Doma had developed such an awful personality was simply because he had chosen a different path from hers. If her goal was to surpass her sister, then Sogo Doma sought to draw people's attention through slacking off—or rather, through pranks...
For example, wasn't the protagonist of Naruto trying to make people notice him through pranks? It might have been childish, but in circumstances like theirs...
Yukino Yukinoshita