I don't know. Both Cassandra and Rock disappeared from the storyline with no clues. The silly girl didn't even save Pyrrha after her sister's death. Now the easily influenced youth hangs around Tira who teaches her bad things, like killing, stealing, smoking, etc.
It was a joke. But Wandrian is right. She did die while trying to remove the shard. But the game did state Tira as the murder. The only way these two could be put together is if Tira was her surgeon.
The funny thing is that Tira wasn't the one who killed Sophitia. In SC4 the last person she faces before the hypathetical Algol fight is Siegfried. She also happens to be the last person he fights before his real battle with Nightmare. There is your culprit.