Personally I think it is ethically questionable because it is not clear to me that their interests won't conflict at some point (like if the parents are charged with accessory after the fact) but it is definitely possible.
As an attorney you have a duty of confidentiality to each client. You can't share what one client tells you with another client.
So theoretically if you are engaged to represent Brian regarding potential criminal charges and then engaged to represent the Laundries to give advice generally, you can have two separate representations and you would have a duty to keep the matters separate and not share information between clients without explicit permission. If your advice to Brian is to not share information with his parents, you could facilitate that by not sharing information yourself either.
I think spelling it out makes it pretty obvious that the situation is messy.
One possible scenario is that Brian refuses to talk to anyone - refuses to talk to his parents, refuses to talk to the attorney. And the attorney is primarily representing the parents without actually knowing what happened either. The attorney can assume it's bad given the blanket advice for NO ONE to speak to LE on the advice of counsel because he just assumes nothing good can come of it. I wouldn't necessarily agree with that advice but his strategy so far has been silence to LE so who knows.