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.
*Disclosure: Some of the links above are affiliate links, meaning, at no additional cost to you, Fandom will earn a commission if you click through and make a purchase. Community content is available under CC-BY-SA unless otherwise noted.