I strongly doubt that DB or JI are involved in the disappearance of their daughter, but I realize it is a possibility. Nothing I've seen so far makes me think so, though.
I think the reason the parents are "not cooperating" (LE's term) is b/c they feel they've said all they know, so they know their non-cooperation is not harming the investigation. IMO their lawyers have pointed out to them that if LE arrests the parents, the investigation is OVER, and that will mean the case will never be solved.
If DB is innocent and has already told everything she knows, then there's nothing else she can say to clear herself. The only thing that can clear her is for LE to follow other leads that hopefully lead to other suspects, and evidence that implicates other suspects.
If DB is innocent and submits to intensive questioning, she is helping LE build a circumstantial case against her. Even if DB's lawyers could beat the charge in a trial, it could irrevocably harm the investigation into what really happened to Lisa.
So I support the parents' decision not to submit to direct questioning by LE, but instead just to answer informational questions through their lawyer, which gives the parents legal protection while still providing LE with needed information.
If DB is guilty IMO there would be physical evidence and witnesses, so LE can prove her guilt without her cooperation.
Of course all MOO!