The first appearance of the word "Sith" in science fiction I have found is from Warlords of Mars by Edgar Rice Burroughs, which appears in these pages.
It seems to be a large insect:
Sith: A giant, venomous hornet-like insect endemic to the Kaolian Forest.
Porges, Irwin; Burroughs, Hulbert; Bradbury, Ray (1975). Edgar Rice Burroughs: The Man who Created Tarzan (1st ed.). Provo, Utah: Brigham Young University Press. p. 163. ISBN 978-0-8425-0079-1.
Burroughs may have derived the name from the Gaelic sith an alternative spelling of sidhe meaning fairy.
The next appearance is if a robot called Sith Tal a robotic museum curator :
