Just playing devil's advocate here, because I think it's unlikely, but it could be that BLs parents are just as in the dark about what happened as we all are. He could have showed up unexpectedly in their driveway sans Gabby and retreated to his room and wouldn't communicate with them or made just vague, incoherent but ominous ramblings. Like everyone else they suspected the worst and called their old lawyer friend in LI who dealt directly with BL and told both BL and the parents that, for the everyone's protection, BL should not discuss any of what happened with the parents and the best way the parents could help BL would be to not ask.
Interesting! To make that hypothesis fit we could consider that maybe Brian left the van at their house (we have no confirmation as to timing), entered the house with his keys. They didn't see him and haven't seen him in all of this time. He then slipped out, taking a set of Mustang keys with him.
They wake up, see the van, don't understand where he is or if Gabby is with him (maybe they had no contact for a while as well?)
(The info gap here is who informed that he arrived on this date and when did LE move in to take van for inspection?)
They have still not seen their son or future daughter in law. Maybe they are getting messages to say that everything is OK and that BL and GP are staying somewhere with friends?
So, continuing with your hypothesis, when the news of Gabby's disappearance breaks, the parents sense something is very wrong and call their lawyer contact, who instructs them to say nothing.
This could be backed up by Brian's sister saying that she has had no contact with him.
The parents see the Mustang is missing and / or receive a message saying that Brian took the Mustang to go for a hike (maybe stated with friends).
They do not worry until he does not return, pressure about Gabby case increases. They think or know that BL disappeared from Park with others helping him and they go to pick up the car. Maybe not knowing or thinking that they were also under the radar?
All the above is mere speculation.
Still fails to address why they did not reply to Gabby's parents or take action much earlier.