The actions of step-parents are relevant evidence in change of custody cases. The step-parent is part of the child's home environment and therefore contributes to that environment - for better or worse.
In this case, the mother lost custody of the children to her ex-husband largely as a result of the actions of her new spouse. The new spouse was abusive towards her including one instance where he intentionally stabbed her with a barbecue fork in the stomach while he was high on methamphetamine - and while she was 8 months pregnant. He also told the children "to enjoy their time with their mother because they were going to living with their daddy forever." She had filed for divorce from him but reconciled with him about a month after the altercation. The trial court felt that this type of behavior put the children in harm's way and subjected them to psychological abuse at a minimum. She divorced the new spouse 2 years later but that was not enough to keep the trial court from taking custody of the children away from her.
She appealed the decision but it was upheld by the Court of Appeals. Here's a link to the decision...http://courts.arkansas.gov/unpublished/2008a/20080618/ca07-419.pdf