"We must've been through hundreds of books already and we can't find him anywhere —"
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Trelawney is the surname of a wizarding family. They have Muggle as well as magical heritage. There is known Seer heritage in the Trelawney family.
Unless Sybill's mother, grandmother, great-grandmother, or Cassandra herself were single mothers or otherwise did not take their husband's name, it is likely that Cassandra Trelawney had a son (Sybill's great-grandfather), who had a son of his own (Sybill's grandfather), who had Sybill's father, in order to account for Sybill to have her great-great-grandmother's surname. Thus, it is also likely that Trelawney was Cassandra's married name.
According to houseofnames.com, the Trelawney family originated in Cornwall.