The sister of a convicted killer who waived his right to appeal his death sentence says they both endured horrific childhood abuse at the hands of their mother.

Cheri Cox was in Boise's U.S. District Court Wednesday to testify in the competency hearing of Joseph Edward Duncan III, who was sentenced to death in 2008 after admitting he kidnapped and tortured two northern Idaho children before killing one of them in Montana.

Duncan gave up his appeals, but his attorneys fought on his behalf and last year an appellate court ordered the competency hearing.

Cox testified about what it was like growing up with Duncan in their childhood home. Cox testified that she and her four siblings were frequently beaten by their mother while she ranted that men were worthless.