Parents and siblings generally have standing to pursue a wrongful death lawsuit. They are usually predicated upon gross negligence. Pointing a gun at someone and then pulling the trigger is grossly negligent if the trigger man did not make sure the chamber was empty at any time before pulling the trigger
Was the murdered daughter a US citizen? As non US citizens in another country, how does that affect their legal standing to sue a US citizen in the US?
Yes, it is right that they should be able to sue the worthless so and so into poverty, but can they?