Great-great-grandmother of Sibyll Trelawney. Cassandra was a “very famous, very gifted” Seer in her day (OP37).
In trying to justify and prove herself, Professor Trelawney tells Professor Umbridge about being descended from Cassandra Trelawney (OP37).
a "very famous, very gifted" Seer in her day (OP37)
Cassandra=Trojan seer gifted with true prophecy, but cursed (by Apollo) so that no one would ever believe her. The people of Troy ignored her warnings not to allow the Trojan Horse within the city's walls, a mistake that allowed the Greeks to capture Troy (EM).
Seemingly irrelevant, the mention of Cassandra Trelawney actually lends enormous support to the idea that her great-great-granddaughter wasn't really a fraud at all (see etymology). -BB