Zora's Domain had been collectively holding their breath since the day Ruta had come to back to life. There was a brief reprieve after it was calmed and Mipha's spirit had guided it into position to take aim at the castle where the Calamity that had claimed her life was trapped, but as the days ticked by, that breath had been held again, tensions riding high.
Each time Sidon traveled to the Zodoban Highlands to try to get a glimpse at the castle, at the open plain that would become a battlefield sooner rather than later, it seemed like both sides were mounting their attack; the cloud of malice would take face and roar its fury before a golden light would shine from somewhere in the castle and the beast would be reined back in. And each time, another beam of light was centered on the castle, from the north, far to the west, and somewhere from the desert to the southwest. One by one, the other Divine Beasts were made ready.
Sidon had been standing there, on the Zodoban Highlands, just below the steep cliff that Ruta had positioned itself in, when the Beasts attacked. Medoh was the first, and then right above his head, his sister unleashed Ruta's fury; the shockwave had nearly knocked him off his feet, and the air felt scorched from the power that aimed itself at Ganon's throat. The light was so blinding that Sidon couldn't even see when the other two Beasts had attacked.
And as abruptly as it started... it stopped. It took a minute before his vision had cleared, but cleared it did, just in time to observe from such a faraway place, that final battle in the fields of Central Hyrule. He was far too far away to see his friend fighting, could only see that monster, evil given form, but he saw its final sealing.
And like that, everything had gone silent. Not even the birds and animals of the land made noise in the wake of that.
Sidon had hoped, foolishly perhaps, that he might get one last glimpse of his sister atop Ruta, before she would inevitably cross over, her duty that had tied her to their world fulfilled. But he hadn't. She was there, and within seconds of Ganon's sealing, she was... gone. He could feel the now-empty place where her spirit had once resided in the land.
But Zora's Domain continued to hold its breath, concerned by the abrupt change in Ruta's behavior, from life back to dormancy. Sidon hardly blamed his people, the Beast had given them something of a case of whiplash, going from silent to rampaging, to standing guard, to a powerful attack, and then back to silence. Sidon could've told them they had no need to fear, that Ruta was silent, possibly for good, but likely only until a new Champion was needed to take up its controls. But explaining how he knew, describing that hollowness that told him that his sister had fulfilled her duty, and with her, Ruta's, and both had finally laid to rest... he wasn't quite sure how to do that.
His father was requesting assistance from Link and the young now-queen Zelda, Sidon was content to let them handle it, and find a place to grieve quietly on his own.
Though he did hope that Link would visit before there was a need for formal ceremony between the queen and Sidon's father. He was the only one that Sidon could think of that he would be comfortable sharing his grief with. Link had known Mipha in a way that their father and the Zora elders never could have- not as a leader, or a student, or a daughter, but as a peer, an equal, a friend. The could have beens were irrelevant at that point. Link had known Sidon's sister as a person, apart from her title, and apart from her role as a daughter. And with him, Sidon felt he could share that loss.