My hunch on this is that LS was in the home with the biological parents, controlling the narrative, putting her arm around BM's shoulders, acting the part of worried stepmom and they were all staying there together (losing precious time since she had lied about everything) until (1) her "interview" which showed her only concern for herself and/or (2) the neighbor found the video. Then AS saw what he had married and what had unfolded and they kicked her out. Think about what was lost simply from the biological parents not ever suspecting her since she was acting the entire time.
She would have had time to plan all of this out - Dad is away in OK for two weeks, probably not even available by cell phone throughout the entire day due to training exercise. May BM is busy working until end of the day Monday when she would normally speak to GS over the telephone in the evening. By that time LS had already reported him as missing and she then has all that time until BM and AS can each fly to Colorado and be there at the home. Meanwhile this person is inside the home, hiding things? arranging the house and the story just so to concoct and make everything fit her superstepmom narrative.
It is really a wonder that some Angel whispered in RD's ear to "check your video surveillance" and this is the prime example of evil only being able to flourish when good people do nothing.