Interrogate the family? You mean, they know something that they are hiding, that would help them find Natalee? You mean they don't really want her found, so they would have to be interrogated to get the information out of them?
How about: Because they know Natalee better than anyone else? Did she know/ have contact with anyone on the island before she went to Aruba? E-mails, chat rooms, IM before she left? Had she called/ msg'd home while she was in Aruba? If so, did she mention anything unusual? Were there any notes, e-mail addresses, etc. in her luggage, in her purse gathered while she was in Aruba? How much money did she take with her? What medications, if any, did she take? For starters.
The parents can provide valuable information for the investigators. I get the impression that you think questioning the parents has to be an adversarial situation. There are certain to be uncomfortable questions like: Do you know or have reason to think that she was sexually active?; Was there friction in the relationship?; How did she get along with her stepdad.... But these questions and others all go to establishing background and it is stuff that investigators need to know. For all we know, the questions may have been asked & answered. OTOH, we know that her purse isn't in the evidence locker at police HQ, so maybe not.
Care to link up the van der Sloots and the Kalpoe brothers with these people who kidnapped her for revenge reasons against her family?
Context is everything. I was replying to another poster and the point was that there may not be a linkage, referring back to Greta's comments about it possibly was a mistake to put all the eggs in one basket. Please backtrack.