A number of lines that Cassandra shouts during her attacks in SoulCalibur VI were first used by Pyrrha or Patroklos in SoulCalibur V. While the ones that come from her nephew are relatively well-known and appear against generic opponents, she only uses her niece's lines when fighting against Sophitia. That might be the reason why their origin still remains quite obscure. Perhaps this split is meant to reinforce that Cassandra only acts meek—Pyrrha-like—towards Sophitia while Patroklos-like attitude is her default mode of behavior toward strangers.
Cassandra's Lines Borrowed from Pyrrha and Patroklos
While we're at it, I think that the translation of the line spoken during Cassandra's Critical Edge isn't that great. Patroklos's is called "The Price of Justice." "圧倒的正義 (あっとうてきせいぎ)" literally means "overwhelming justice" or something along those lines. The official translation is not entirely accurate, but still fitting. I can't say the same about Cassandra's Critical Edge in SoulCalibur VI. The translator had to include "price" in there as well in order to preserve the reference, but it led to Cassandra's phrase becoming "price of integrity", losing almost all connection with the original. Her original Japanese phrase is "圧倒的制裁 (あっとうてきせいざい)," which means "overwhelming sanctions"/"overwhelming punishment". It places more emphasis on getting back at someone and personal justice.
While I'm not a fan of the official translation, I acknowledge that there was probably no good way for VI's translators to follow up on V's line. These lines take advantage of Japanese in a way that makes them hardly translateable at all.