Sheila Clare Ryan, 57, assistant manager of the Smithsonian Institution Museum Shop and a former potter and ceramicist, died of cancer Tuesday at Georgetown University Hospital.

Born in Lincoln, Neb., she grew up in Washington and in Massachusetts. She graduated from the Convent of the Sacred Heart in Washington and attended the University of Iowa, the Leland Powers School of the Theater in Boston and the University of California at Los Angeles.

During World War II, she was a member of a USO comedy troupe that toured the Eastern Seaboard. She subsequently worked in radio advertising for a Washington radio station and the Enders Advertising Agency here.

From 1963 to 1978, Miss Ryan lived in California, where she worked as a potter and ceramicist. She then returned to Washington and subsequently joined the Smithsonian.

A sister of Congressman Leo J. Ryan (D-Calif.), she was an outspoken opponent of religious cults and recently made several television appearances calling for a full investigation of the congressman's murder two years ago in Jonestown, Guyana. The congressman and four members of his party were killed in an ambush in Jonestown by followers of religious cult leader the Rev. Jim Jones.

Miss Ryan lived in Washington. Her marriage to Nicholas E. Devereux ended in divorce.

Survivors include a daughter, deirdre Ann Devereux of Venice, Calif.; three sons, Nicholas Devereux of Van Nuys, Calif., Ramsay Devereux of Encino, Calif., and Drew Devereux, also of Venice; her mother, Autumn Mead Ryan, and a sister, Deirdre M. Ryan, both of Woodstock, Conn., and another sister, Shannon R. Torphy of Colts Neck, N.J.

The family suggests that expressions of sympathy be in the form of contributions to the American Cancer Society.