Thank you for opening up the discussion to other possible scenarios. I just have the gut feeling that this is not about purely a rape/murder situation. Here is something to consider..
Sierra mentioned in a tweet, shortly before she disappeared that she thought it was weird that people she knew from Fremont were friends with people who went to her school. (Follow me on this one) Her father was charged with molesting someone approx the same age as her sister, years ago. I am not certain about her sister's age, but I thought it was around 20 (please correct me if I am wrong). So Sierra moves from Fremont to Morgan Hill and just discovers she knows someone in Morgan Hill who has a mutual friend in Fremont. What if the girl who was molested has some connection to the Morgan Hill friend and finds out that Sierra now lives in Morgan Hill. Perhaps a boyfriend who sees the long term effects of the molestation on his girlfriend and decides to retaliate by harming Sierra. Take it one step further and consider the possibility that this person has some connection to a gang. Maybe this is not about rape, but about retaliation. Maybe Antolin is one of the people given the job of handling this affair, perhaps with a promise of monetary reward.
I just do not think that Antolin kidnapped Sierra for the purposes of rape. I really feel this is something far more complicated.
Just something to consider, and of course nothing more than speculation or theorizing about other possible motivations.