Retired oilman Howard B. Keck and his two daughters filed suit against his sons and 19 other family members in an attempt to establish Keck's adopted daughter as a legitimate heir to his multimillion-dollar estate. In a petition filed in Los Angeles Superior Court, the 72-year-old retired chairman of Superior Oil Co. said he fears that his sons may try to thwart their half-sister's inheritance "based on purely selfish, monetary and improper motives" unless Keck guarantees her rights before his death. Frances Elizabeth Drown was 5 years old when her mother, Elizabeth, married Keck in 1950, and Keck said she has been as much his daughter as the three children Elizabeth bore after their marriage. A formal adoption petition is pending, and Keck's natural daughter, Erin Anne Lower, is supporting Keck's attempt to include Frances and her children among her father's heirs. Drown's mother, who has filed for divorce, had "until recently . . . always assented to and agreed to the adoption," the suit said.