Here is my theory: Joran Van Sloot accidently killed Natalee Hollaway and the Kalpoe brothers were present when it happened. Knowing Van Sloot's track record, he was obviously expecting a one-night-stand. Possibly put Extasy in her drink and she had a fatal reaction, or perhaps they got into a scuffle when he made a pass at her and he fatally injured her. Then the three panicked and disposed of her body, then concocted a lie to cover it up.
Until Natalee is found, it is my theory is that she is still alive being forced to do things she would not normally do.