If powerful forces murdered theie son, those same forces would threaten the parents and siblings.
That should be the assumption going forward that they have been threatened to play along or else.
The best way to assist them is to investigate and free them of the threats by discovering the truth.
You nailed it.
I am operating on the assumption that they have been threatened. Therefore, the best course of action is to ignore what they are saying and assume they are compromised.
Investigate. Investigate. Investigate. The truth will set them free.