מדרש על סנהדרין 54:16

From (Devarim 24:16) "Fathers shall not be put to death by cause of sons, and sons shall not be put to death by cause of fathers," we learn that they (i.e., kin) do not testify against each other. We hereby infer (that to secure testimony against an inciter to idolatry) witnesses are sequestered against him.