Could victim’s family sue and force USSS and FBI to give all evidence in a court?
I'm sure that ANY EVIDENCE would MYSTERIOUSLY DISAPPEAR or ACCIDENTALLY be DESTROYED before ever being released to the public to discover the TRUTH... similar to the situation with Hillary Clinton's EMAILS and SERVER.
“Could victim’s family sue and force USSS and FBI to give all evidence in a court?”
They can sue, but the case would not survive a motion to dismiss because — like LEO — the SS owes no duty of care to the general public. (FYI: Negligence requires a breach of duty, and in the absence of a duty of care, there cannot be negligence even if the damages are horrific.)