A Burt County district judge this week denied a request to move the 15-year-old boy's case to juvenile court.
Wilcox faces charges of first-degree murder, arson and forcible sexual assault.
Prosecutors say Wilcox sexually assaulted his 4-year-old half-sister, smothered her and then set a fire at the family's home on May 1.
He escaped from the flames with his mother and another sister.
Wilcox's attorney has said he has the mental capacity of a second-grader.
A hearing in the case is set for Feb. 3.
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