ALFRED KASSAB, 73, whose persistence led to the conviction of his son-in-law, Dr. Jeffrey R. MacDonald, in the 1970 murder of the doctor's wife and two young daughters, died Oct. 24 in Rockledge, on Florida's Atlantic coast, of emphysema. The case became the subject of Fatal Vision, a widely read book and a two-part television movie. At first, Mr. Kassab was a strong supporter of his son-in-law, who contended that his pregnant wife, Colette, 26, and daughters Kimberly, 5, and Kristen, 2, had been slain by drug-crazed intruders. The murders took place at the family's home at Fort Bragg, N.C., where MacDonald was a Green Beret captain and Army physician. After studying transcripts of Army hearings on the deaths, Mr. Kassab became convinced of MacDonald's guilt and began his crusade.