If they had an efficient workable law system the family would have had NO effect on their performance. AND finding the body would have helped tremendously, although there is no doubt she has been murdered.
Dutch law is Dutch law. When in someone else’s country you play by their rules.
The parents went in and successfully stalled the investigation via interviews in the media and criticism of local officials. It’s not difficult to see what happened, the cops basically took a vacation until the controversy died down and then went back to work on the case.