It depends how you interpret this sentence.
“The assaults took place at both the victim’s Lake Elsinore home and at church events at the local LDS Church, in which both her mother and stepfather were active, the lawsuit said.”
They were both active in the church or active church members. Which is normal church speak. Elsewhere it clearly states that assaults continued even after the mother had knowledge of them. Generally that means the mom dithered before contacting authorities or failed to contact them at all. But the stepfather was arrested, so somebody eventually reported the sob.
“active participant” meams active.