BOSTON (AP) _ Tobacco heiress Carol Lynn Benson Kendall, who survived a car bombing that killed her mother and adopted brother, has said under oath that the dead young man was actually her son, her attorney said today.

Florida authorities have charged Steven Benson, Mrs. Kendall's 34-year-old natural brother, with setting the blast outside the family's Naples, Fla., home because he believed he was about to be cut out of his mother's will.

During a deposition taken by Benson's attorneys in Florida on Sept. 20, Mrs. Kendall, 40, said she was an unmarried teen-ager when she became pregnant, The Boston Herald reported today. Mrs. Kendall's attorney, E. Richard Cirace of Boston, confirmed the report.

Cirace said Scott was born in 1965 in a Baltimore hospital and later adopted by Mrs. Kendall's mother, Margaret Benson of Pennsylvania, heiress to the Lancaster Leaf Tobacco Co. fortune. Mrs. Benson was 63 when she died in the blast.

''There had been general rumors (about the boy's parentage) going around Lancaster, where the family was brought up,'' Cirace said. ''People had suspicions as to why, after having two children of her own, all of a sudden Mrs. Benson adopts a third.''

Cirace said the rumors were never confirmed until Steven Benson's lawyers questioned Mrs. Kendall about it, to test her credibility. ''It wasn't discussed. I guess people just exhibited good taste,'' he said.

He said Mrs. Kendall, who was recovering from severe burns at her home in the Chestnut Hill section of Boston, recently told her teen-age sons about Scott's parentage, but never told Scott.

''To my knowledge, Scott never confronted his mother with the question. The family and everybody just treated his grandmother as Scott's mother,'' the attorney said.

''I have a strong feeling Steven Benson also knew,'' Cirace said. ''We're going on the premise Scott did not know, but Steven, being the older of the two boys - I have strong feelings he did know.''

He said he counseled Mrs. Kendall to tell the truth about Scott's birth and ''answer any and all questions they ask, no matter how private or embarrassing.''

Steven Benson's trial is scheduled to begin in Naples, Fla., on Nov. 13.